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To change the SMTP envelope sender (the address in the MAIL FROM command), configure sender_canonical_maps, rebuild its lookup table with postmap, and reload Postfix. If the visible From: header must stay unchanged, set sender_canonical_classes = envelope_sender. Use smtp_generic_maps instead when the rewrite should happen only as mail leaves the host through the SMTP client.
The envelope sender and the visible From: header are separate address layers. The envelope sender controls delivery and bounce handling; the receiving system commonly records it as the message’s Return-Path.
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What you are changing: envelope sender, header sender, and Return-Path
SMTP carries a message with an envelope. Its sender is supplied in the MAIL FROM command and is used by receiving servers for delivery status notifications and bounces. The message content also contains headers, including the visible From: field that mail clients display. Changing one does not automatically change the other.
A Return-Path header is normally added by the receiving system from the envelope sender. Setting a visible From: value therefore does not change where bounces go. To verify the envelope value, inspect the receiving server’s SMTP transaction or the delivered message’s resulting Return-Path; do not rely only on what a mail client displays in the From: line.
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This distinction matters in several common situations. You may want a local account such as olduser@example.invalid to continue appearing in the message header while bounces are routed to newuser@example.com. You may instead need both the envelope and headers changed, or you may want the original address preserved inside Postfix and changed only on external delivery. Each requirement points to a different mapping stage.
Choose the Postfix mechanism first
The right setting depends on where the rewrite belongs in Postfix’s processing path and which address layers it may change.
| Mechanism | Processing stage | Addresses affected | Typical use |
|---|---|---|---|
sender_canonical_maps |
Postfix cleanup address mapping | Envelope sender and, by default, header sender; controlled with sender_canonical_classes |
Change a sender identity as Postfix processes the message, including an envelope-only rewrite |
smtp_generic_maps |
Outbound SMTP delivery | Envelope and header addresses during SMTP delivery | Transform local or private addresses only when mail leaves this host |
Use sender canonical mapping for an envelope-only change
Choose sender_canonical_maps when the sender rewrite is a cleanup-stage policy. Its default classes are envelope_sender, header_sender, so a basic configuration can change both the SMTP envelope sender and sender addresses in headers. Add sender_canonical_classes = envelope_sender when the requirement is specifically “change MAIL FROM, keep From: unchanged.”
This sender-focused map is also the better fit when you want the mapping to be applied as Postfix handles the message rather than waiting for a particular outbound SMTP transport. Sender-specific mappings are applied before common canonical_maps, which helps keep a sender-only policy from becoming an unintended recipient rewrite through a broader canonical rule.
Use a generic map for outbound-only transformation
Choose smtp_generic_maps when the address should be transformed only as Postfix’s SMTP client delivers mail. This is useful for converting a local-only domain into a publicly deliverable address at the outbound boundary. The generic mapping affects both envelope and header addresses and is non-recursive, so it is not an envelope-only substitute for a sender canonical map.
Because generic mapping occurs during outbound SMTP, the address retained during earlier queue and cleanup processing can differ from the address presented to the remote server. That boundary-specific behavior is useful when internal processing should continue to use the local identity, but external recipients must see and receive a routable address.
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This method is the usual choice when you need a sender-specific rewrite and want explicit control over whether headers are included.
1. Select a supported lookup-table type
Open /etc/postfix/main.cf and add a sender map. A documented example uses an LMDB table:
sender_canonical_maps = lmdb:/etc/postfix/sender_canonicalYour installation may use another supported table type, such as hash:, if that database support is available. The database prefix and source path must match the tools and table files installed on your system. If the prefix in main.cf says lmdb:, use the corresponding source path with postmap; if it says hash:, build the hash table from that same source path.
2. Create the source mapping
Create /etc/postfix/sender_canonical. Put the address Postfix currently sees on the left and the replacement address on the right, separated by whitespace:
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olduser@example.invalid newuser@example.comThe left-to-right direction matters. Keep entries narrowly scoped to the senders that should change, and check the file for unintended broad or wildcard entries before activating it. A mapping that is too broad can affect more messages than the one account or domain you intended to alter.
Use the exact form of the sender that appears at the point where the map is applied. For example, if the message is submitted as olduser@example.invalid, an entry for a different spelling or domain will not express the same policy. Keep one mapping per line and preserve the source file as the human-readable record of the intended rule.
3. Decide whether headers should change
Without an additional setting, sender canonical mapping applies to both envelope sender addresses and header sender addresses. For envelope-only rewriting, add this line to main.cf:
sender_canonical_classes = envelope_senderWith that class selection, the SMTP MAIL FROM value is rewritten while the visible sender address in message headers is left as it was. If both layers should change, leave the default classes in effect rather than restricting them. The default is effectively envelope_sender, header_sender.
Do not infer the class setting from the map file itself. The same left-to-right entry can produce different results depending on the active classes. Check the effective value with postconf before testing, especially on a host whose configuration is assembled from multiple files or package defaults.
4. Build the indexed table
After creating or editing the source file, compile it with postmap:
postmap /etc/postfix/sender_canonicalRun this command after every edit. Postfix reads the generated lookup database, not merely the text file, so changing the source without rebuilding leaves the active map out of date. Run the command against the exact source path named in the map declaration; building a similarly named file elsewhere does not update the table Postfix is using.
5. Reload Postfix
Make the configuration visible with:
postfix reloadA reload activates configuration changes without waiting for Postfix’s normal configuration-delay behavior. It is the final step after changing main.cf or rebuilding the map. If you change both the declaration and the source mapping, rebuild first and then reload so the running process sees the complete change.
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How sender canonical mapping interacts with other canonical maps
Sender-specific canonical mappings are applied before common canonical_maps. That ordering is useful when the requirement is limited to sender rewriting: the sender map can handle the intended sender change without turning the same policy into an unintended recipient rewrite through a broader canonical map.
When diagnosing an unexpected result, therefore, review both the sender-specific setting and any common canonical configuration. A sender entry may be correct while a later or broader rule changes another address layer. Keep the sender map narrowly focused when the stated requirement is only to alter the envelope sender.
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Method 2: rewrite at outbound SMTP with smtp_generic_maps
Use this method when the original address is appropriate inside the host but must be changed at the point of external SMTP delivery.
1. Configure the generic map
Add the map declaration to /etc/postfix/main.cf:
smtp_generic_maps = hash:/etc/postfix/genericThe example uses a hash: table. Use a table type supported by your Postfix installation if your local setup differs. As with the sender map, the table prefix and source path in main.cf must correspond to the database generated from the source file.
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Create /etc/postfix/generic with the local address on the left and the externally deliverable address on the right:
olduser@localdomain.local newuser@example.comGeneric mapping is an outbound transformation. During SMTP delivery it replaces both envelope and header addresses, and the mapping is non-recursive: Postfix does not repeatedly feed the replacement back through the same map. This makes the rule suitable for a one-time boundary conversion, but it also means you should define the final desired value on the right-hand side rather than expecting a chain of substitutions.
3. Compile and reload
Rebuild the indexed table and reload Postfix:
postmap /etc/postfix/generic
postfix reloadRebuild after every source-file edit. A reload is still required after changing the configuration declaration in main.cf. If only the source mapping changed, the reload ensures the running service reopens or recognizes the updated lookup data according to the active configuration.
When this method is not appropriate
If the visible From: header must remain the original address while only MAIL FROM changes, do not use a generic map for that purpose: its documented scope includes both envelope and header addresses. Use sender_canonical_maps with sender_canonical_classes = envelope_sender instead.
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Configuration examples by requirement
| Requirement | Configuration | Result |
|---|---|---|
| Rewrite envelope sender and sender headers during cleanup | sender_canonical_maps = lmdb:/etc/postfix/sender_canonical; leave the default classes |
The mapped sender is changed in the envelope and sender-related headers |
Rewrite only MAIL FROM |
sender_canonical_maps = lmdb:/etc/postfix/sender_canonicalsender_canonical_classes = envelope_sender |
The envelope sender changes while the visible sender header remains unchanged |
| Rewrite addresses only at external SMTP delivery | smtp_generic_maps = hash:/etc/postfix/generic |
Outbound SMTP delivery uses the mapped value for envelope and header addresses |
Example decision sequence
- Write down the address that must be presented in
MAIL FROM. - Decide whether the message’s sender headers must remain unchanged, change with the envelope, or change only at the outbound boundary.
- Choose
sender_canonical_mapsfor cleanup-stage sender policy, orsmtp_generic_mapsfor outbound SMTP transformation. - Choose the supported table type and keep its source path consistent in the declaration, source file, and
postmapcommand. - Rebuild the table, reload Postfix, and test the receiving side.
Verify the active configuration before testing
Check which mechanism Postfix is actually configured to use:
postconf sender_canonical_maps sender_canonical_classes
postconf smtp_generic_mapsThese commands show the effective values, which is more reliable than assuming a particular line exists in a configuration fragment. Confirm that the intended map path and table type appear in the output. If the output is empty or points to another file, correct the active configuration before changing the map contents.
Inspect the source entry
- Confirm that the original sender is on the left and the replacement is on the right.
- Confirm that the file named in
main.cfis the file you edited. - Look for an unintended wildcard or broad entry that could rewrite other senders.
- Run
postmapagainst that exact source path after the last edit. - Check the active sender classes so you know whether header senders are included.
Send a controlled test
Send a test message from the address that appears on the left side of the map. Inspect the receiving server’s SMTP transaction, the delivered message’s headers, or its resulting Return-Path. The important observation is the envelope value accepted in MAIL FROM, not just the header rendered by a mail client.
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There is no single universal external verification command prescribed for every deployment. A controlled message and inspection at the receiving side directly tests the behavior that matters. For an envelope-only test, compare the accepted MAIL FROM or resulting Return-Path with the delivered From: header; the two should differ when that is the intended result.
Troubleshoot a rewrite that appears not to work
The map was edited but the old address is still used
- Run
postconf sender_canonical_maps sender_canonical_classesorpostconf smtp_generic_mapsto confirm the active setting. - Run
postmapagainst the exact source file used by the map declaration. - Run
postfix reloadafter rebuilding the table. - Send another controlled test and inspect the envelope transaction or resulting
Return-Path.
Editing only the text source does not update the indexed lookup database; omitting the reload leaves the running configuration unchanged. Also verify that the test message actually traverses the Postfix service and processing stage where the selected map applies.
The visible From header changed unexpectedly
Check sender_canonical_classes. Its default includes both envelope_sender and header_sender. Set it explicitly to envelope_sender when the header must remain untouched, then rebuild any changed map if necessary and reload Postfix. Test both the SMTP envelope and the delivered header afterward, because correcting one layer does not prove the other is correct.
One service ignores a map that works elsewhere
Inspect the relevant service in master.cf. A service can set receive_override_options=no_address_mappings, which disables address mappings for that service. A global map can therefore appear to work for one delivery path but not for a service with this override. Compare the service that succeeds with the service that bypasses the rewrite, and check for this option before changing otherwise-correct map entries.
The wrong address layer was tested
A mail client showing the desired From: value does not prove that the envelope sender changed. Conversely, a changed bounce address does not prove that the visible header changed. Test each layer separately: inspect the SMTP MAIL FROM value for the envelope and inspect the message headers for the displayed sender.
The chosen mechanism changes more than intended
Review the processing stage and scope. A generic map is deliberately applied during outbound SMTP and affects envelope and header addresses. A sender canonical map is applied during cleanup and can include headers unless its classes are restricted. Select the mechanism whose stage and address scope match the requirement rather than trying to compensate with a header-only edit.
The map works for one path but not another
Compare the path’s processing stage and service settings. A cleanup-stage sender map and an outbound-only generic map are not interchangeable. A service-level receive_override_options=no_address_mappings can bypass mappings even when the global configuration and table contents are correct. Verify the active map declarations and the relevant master.cf service before broadening the rule.
Operational checklist
- Define the exact outcome: envelope only, envelope plus headers, or outbound-only transformation.
- Choose
sender_canonical_mapsfor cleanup-stage sender rewriting, orsmtp_generic_mapsfor outbound SMTP transformation. - Use a precise left-to-right mapping in the corresponding source file.
- Set
sender_canonical_classes = envelope_senderif the visible sender header must not change. - Check the effective values with
postconf. - Run
postmapagainst the exact source file after every edit. - Run
postfix reload. - Send a controlled test and verify the receiving server’s envelope transaction, headers, or resulting
Return-Path. - If a path still bypasses the rewrite, inspect its
master.cfservice forreceive_override_options=no_address_mappings.
Frequently overlooked details
Changing From: is not changing MAIL FROM. Header rewriting alone does not redirect bounces. Confirm the envelope independently.
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The default sender classes are broader than envelope-only. Unless you set sender_canonical_classes = envelope_sender, sender canonical mapping can also rewrite sender addresses in headers.
Generic mapping is not recursive. Put the final outbound address on the right side of the generic table; do not expect successive entries to be applied repeatedly.
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Rebuilding and reloading are separate operations. postmap updates the indexed lookup data, while postfix reload makes configuration changes visible to the running service. Perform both when appropriate.
Service overrides can explain selective failures. If one service ignores a map, inspect master.cf for receive_override_options=no_address_mappings before changing the global policy.
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Frequently Asked Questions
How do I change only the Postfix envelope-from address?
Configure sender_canonical_maps, create a left-to-right sender mapping, run postmap, and reload Postfix. Set sender_canonical_classes = envelope_sender so the visible sender headers are not rewritten.
How do I change the Return-Path in Postfix?
Change the SMTP envelope sender supplied in MAIL FROM. The receiving system normally creates Return-Path from that envelope value; changing only the visible From: header does not change bounces.
Should I use sender_canonical_maps or smtp_generic_maps?
Use sender_canonical_maps for cleanup-stage sender rewriting, especially when you need envelope-only control. Use smtp_generic_maps when the transformation should occur only during outbound SMTP delivery and may affect both envelope and header addresses.
Will sender_canonical_maps change the From header too?
By default, its classes include both envelope_sender and header_sender, so it can. Set sender_canonical_classes = envelope_sender to restrict the rewrite to the envelope sender.
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Yes. Run postmap against the exact source path after every edit. Postfix uses the generated indexed table, so changing only the text file leaves the active lookup data unchanged.
Is postfix reload required after changing a mapping?
Run postfix reload after changing main.cf or rebuilding the map. Reload makes the configuration visible without waiting for the normal configuration delay.
Why does smtp_generic_maps change both From and Return-Path?
Generic mapping is an outbound SMTP transformation whose documented scope includes both envelope and header addresses. It is therefore unsuitable when only MAIL FROM should change.
Why does a configured map work for one service but not another?
Inspect the service definition in master.cf. A service with receive_override_options=no_address_mappings disables address mappings, so it can bypass a map that works on another path.
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Send a controlled test and inspect the receiving server’s SMTP transaction for MAIL FROM, or inspect the delivered message’s resulting Return-Path. Check the message headers separately to verify the visible From: value.
The Bottom Line
For a sender-specific envelope rewrite, use sender_canonical_maps, rebuild the table with postmap, and run postfix reload. Restrict it to sender_canonical_classes = envelope_sender when the visible From: header must stay unchanged. Choose smtp_generic_maps only for an outbound SMTP transformation that should affect both envelope and header addresses, and verify the result at the receiving server.
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