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A reliable VPS mail server is a chain of three services: Exim4 accepts and transfers SMTP mail, Dovecot provides IMAP or POP3 access to stored messages, and ClamAV scans message content for malware. Build the chain in that order, give the VPS a correct fully qualified domain name (FQDN), protect every authenticated SMTP session with TLS, and test locally before trusting the server with production mail.
This guide uses Debian or Ubuntu conventions and shows a maintainable split Exim configuration, Dovecot mailbox access, and an Exim-to-clamd virus-scanning policy. Package names and paths can vary slightly by supported release, so read the generated configuration and service status after each change.
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What each service does
Mail delivery is not one operation. A mail user agent (for example, a desktop or mobile mail app) submits a message to a mail transfer agent (MTA). The MTA transfers it to another MTA or to a local delivery agent, which writes it to a mailbox. The recipient’s mail app then retrieves it through IMAP or POP3.
| Component | Role in this setup | What it does not provide |
|---|---|---|
| Exim4 | SMTP submission, reception, routing and transfer | It is not the mailbox-reading service for users. |
| Dovecot | IMAP and POP3 access to messages stored by the delivery system | It does not replace Exim as the SMTP transfer agent. |
| ClamAV | Scans message content for malware through the clamd daemon |
It is not a spam-filtering, reputation or deliverability system. |
Keeping these boundaries clear makes troubleshooting easier. An SMTP connection problem belongs first to Exim and the firewall; a mailbox-login problem belongs to Dovecot; a virus rejection problem belongs to the ClamAV socket, permissions and Exim ACL.
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Prepare the VPS before installing mail software
Use a supported operating system and stable hostname
Start with a supported Debian or Ubuntu release and a VPS provider that permits mail traffic. Some providers block outbound SMTP, especially TCP port 25, until you request an exception. Confirm that policy before spending time on configuration.
Choose a permanent host name such as mail.example.net and set it as the system’s FQDN. The name used by Exim for TLS must match the system FQDN; Ubuntu’s Exim documentation warns that the “Server name” setting is the most important value because TLS will not work correctly when it is wrong. Use the same public name when obtaining the production certificate.
Plan the mail domain and DNS
Decide which domain the server will handle, for example example.net, and which host will receive its mail, such as mail.example.net. Create an address record for the host and an MX record for the mail domain that points to that host. Also plan reverse DNS (PTR) with the VPS provider. Forward and reverse names that agree are important for reputation, but they are not configured by Exim or Dovecot themselves.
Before opening the service to users, publish and verify the domain’s sender-authentication records (SPF, DKIM and DMARC) according to your chosen signing and policy design. The procedures below validate local service wiring; they do not prove that remote providers will accept your mail or that your DNS policy is correct.
Decide how mail will leave the VPS
| Routing model | Control | Operational burden | When it fits |
|---|---|---|---|
| Direct delivery from Exim | Maximum control over SMTP routing and queueing | You manage reverse DNS, reputation, rate limits, TLS policy and remote-provider blocks. | Teams prepared to operate internet-facing mail infrastructure. |
| Smarthost relay | The relay provider handles onward delivery and much of its reputation | You depend on that provider’s credentials, limits and availability. | Small deployments that want simpler outbound delivery. |
The Ubuntu and Debian configuration mechanisms do not select a universal choice. Make this decision before the Exim wizard so its system-wide routing mode matches your plan.
Install and configure Exim4
Install the package and choose a configuration layout
- Refresh package metadata and install Exim4 with your distribution’s package manager. On Debian or Ubuntu this is typically
sudo apt updatefollowed bysudo apt install exim4. - Run
sudo dpkg-reconfigure exim4-configto launch the Debian configuration wizard. - Choose split configuration if you want modular files under
/etc/exim4/conf.d. Choose the single-template layout if you deliberately prefer one generated template. Split files are easier to isolate and maintain as the ACL and transport rules grow; a single template can be simpler for a very small, tightly controlled installation. - Enter the system mail name, the FQDN and the domains for which this server accepts mail. Select direct delivery or smarthost behavior to match the routing decision above.
- Start Exim with
sudo service exim4 startif the service is not already running.
Do not edit generated files blindly. The wizard writes the base values, and later changes should be made in the appropriate source file before regenerating the active configuration.
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Enable authenticated SMTP
Submission clients need authentication so an internet-facing server cannot be used as an open relay. Exim can authenticate through PAM-backed saslauthd or through an Exim password database. Select one method and document where its credentials come from.
Both methods can expose clear-text passwords on the wire if a client connects without TLS. Treat authentication and encryption as one requirement: do not permit users to submit credentials over an unencrypted connection. Configure the client submission service to require TLS, then test an authenticated login only after the certificate and protocol settings are working.
Configure Exim TLS
- Install a certificate whose subject name covers the mail FQDN. A self-signed certificate is useful for initial wiring tests, but production clients and remote MTAs should receive a certificate issued by a trusted authority.
- Set the Exim TLS option
MAIN_TLS_ENABLE = yesin the appropriate configuration source. - Ensure the certificate and private-key paths are readable by the Exim process while the private key remains protected from unprivileged users.
- Regenerate the active configuration with
sudo update-exim4.conf. - Restart Exim and inspect its service status and logs before testing a client connection.
The certificate name and Exim’s “Server name” must agree with the FQDN. A mismatch commonly appears as a client certificate warning or a failed TLS negotiation even though the SMTP port is reachable.
Install Dovecot for mailbox access
Install IMAP and POP3 support
- Install the protocol packages:
sudo apt install dovecot-imapd dovecot-pop3d. - Review
/etc/dovecot/dovecot.confand the files in/etc/dovecot/conf.d/rather than assuming the package defaults match your mailbox design. - Choose either mbox or Maildir deliberately. The delivery format selected in Exim must be the same format Dovecot reads.
- Restart Dovecot after changing its configuration.
Choose mbox or Maildir
| Format | Mailbox layout | Operational implication |
|---|---|---|
| mbox | Messages are stored together in a mailbox file. | File locking and large-file handling become important as the mailbox grows. Select it only when your delivery and backup procedures are designed for that layout. |
| Maildir | Messages are stored as individual files in a directory tree. | Per-message files can simplify incremental handling and some recovery tasks, but backups and file-count limits must be planned. |
Neither format is universally best. The critical requirement is consistency: Exim’s local delivery transport, Dovecot’s mail location and your backup procedure must all describe the same layout.
Use the documented client ports
Ubuntu Server documentation lists these Dovecot ports (2026):
| Protocol | Port | Encryption model |
|---|---|---|
| IMAP | 143 | Plain protocol port; use it only with a TLS upgrade policy such as STARTTLS. |
| IMAPS | 993 | TLS-wrapped IMAP from the first byte. |
| POP3 | 110 | Plain protocol port; use it only with a TLS upgrade policy such as STARTTLS. |
| POP3S | 995 | TLS-wrapped POP3 from the first byte. |
Ubuntu states that “IMAPS and POP3S are more secure because they use SSL encryption to connect.” For new clients, prefer the TLS-wrapped ports unless you have a specific reason to use STARTTLS on 143 or 110. Open only the ports you actually operate in the VPS firewall.
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Configure Dovecot TLS
- Open Dovecot’s SSL settings, normally in
/etc/dovecot/conf.d/10-ssl.conf. - Point the certificate and private-key settings at the trusted certificate for the mail FQDN.
- Set a policy that prevents password authentication from being accepted on an unencrypted connection. The exact directive names depend on the supported Dovecot release, so validate the generated configuration rather than copying a setting meant for another version.
- Restart Dovecot and test both a successful TLS connection and a deliberately rejected insecure login.
Certificate renewal is part of operations. Replace the certificate before expiry, reload or restart Dovecot and Exim as required, and verify that clients see the renewed name and chain.
Add ClamAV malware scanning
Install the scanner and signature updater
- Install the scanner, daemon and updater packages:
sudo apt install clamav clamav-daemon clamav-freshclam. - Keep the signature database current with
clamav-freshclam. Debian documentation identifies this component as the database updater andclamav-daemonas the scanning daemon. - Start or restart the daemon after installation and confirm that its Unix socket exists.
ClamAV scanning adds content inspection to the SMTP path. It does not replace authentication, TLS, spam controls, backups or a policy for handling false positives.
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On a split Exim configuration, set the scanner endpoint to:
av_scanner = clamd:/var/run/clamav/clamd.ctl
Then add a DATA ACL that rejects a message when Exim’s malware result is nonempty. The important behavior is the condition, not a particular rejection sentence: a message identified by ClamAV should be refused during SMTP processing rather than silently delivered.
- Place the scanner setting in the appropriate split configuration file.
- Add the malware check to the DATA ACL, using a rule equivalent to “reject when
malware = *”. - Add the
clamavuser to theDebian-eximgroup so the scanner can access the Exim spool as required by the integration. - Restart
clamav-daemon, regenerate the Exim configuration withsudo update-exim4.conf, and restart Exim. - Check both services’ logs for socket, permission or protocol errors.
The socket path, ACL and permissions are a single dependency chain. A correct ACL cannot scan anything if clamd is stopped; a running daemon cannot scan if Exim cannot reach /var/run/clamav/clamd.ctl; and a reachable socket still fails when the service account lacks the required spool access.
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Permit only the traffic your design needs. A typical direct-delivery server needs SMTP on TCP 25 for MTA-to-MTA transfer, an authenticated submission service for users, and the selected Dovecot ports. If you expose both encrypted and plaintext protocol ports, enforce STARTTLS and authentication rules on the plaintext ports rather than treating a firewall opening as encryption.
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- SMTP transfer: TCP 25.
- IMAP: TCP 143, normally with a TLS-upgrade policy.
- IMAPS: TCP 993.
- POP3: TCP 110, normally with a TLS-upgrade policy.
- POP3S: TCP 995.
The exact authenticated-submission port is determined by your Exim configuration and client policy; do not advertise a port you have not configured and tested. Restrict administrative access, keep package updates current and monitor disk space, queue growth, certificate expiry and service restarts.
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Check service state and generated configuration
- After every restart, inspect Exim, Dovecot and ClamAV service status.
- Regenerate Exim’s configuration after editing source files and verify that the active configuration contains the TLS and scanner settings.
- Confirm that the Dovecot certificate paths exist and that the private keys have restrictive permissions.
- Confirm that the ClamAV socket exists at
/var/run/clamav/clamd.ctland that the service accounts have the required access. - Watch logs while performing each protocol test so a failed client action can be matched to a server-side error.
Test POP3 locally
Ubuntu’s Dovecot guide demonstrates a localhost protocol check with Telnet. Connect to the POP3 listener on port 110 and expect a greeting such as +OK Dovecot ready. This confirms that Dovecot is listening and speaking POP3; it does not prove that TLS, authentication or external firewall rules are correct.
Repeat the check against the TLS-wrapped service with a TLS-capable client, and test an actual mailbox login using a non-production account. Verify that a message delivered by Exim appears in the mailbox format Dovecot is configured to read.
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Test SMTP submission and delivery
From a controlled client, establish TLS, authenticate, submit a message and confirm that Exim queues or delivers it according to your routing mode. Test a local recipient first. For direct delivery, then test a recipient at an external provider and inspect the remote response, headers and any bounce. A successful local submission does not establish internet-wide deliverability.
Run the EICAR antivirus test
The Exim/ClamAV integration guidance recommends sending the standard EICAR test string as an attachment or message body from a controlled test account. A correctly wired server should reject it immediately during SMTP processing. Remove the test message and confirm that ordinary, non-test mail still passes.
This test validates scanner wiring and the ACL decision. It does not measure detection of every malware family, spam performance or remote-provider acceptance.
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Troubleshoot by symptom
Clients cannot establish TLS
- Verify that the certificate name matches the system FQDN and the hostname users enter.
- Check that Exim has
MAIN_TLS_ENABLE = yesand that the generated configuration was rebuilt. - For Dovecot, verify the paths in
10-ssl.conf, certificate readability and a completed service restart. - Check the VPS clock; an incorrect clock can make an otherwise valid certificate appear invalid.
Authentication fails or credentials appear exposed
- Confirm that the selected PAM or Exim password-database method is enabled and points to the intended account source.
- Do not troubleshoot credentials by disabling TLS. Authentication methods can use clear-text passwords unless TLS protects the connection.
- Test with a dedicated account and rotate it after troubleshooting.
Dovecot accepts a connection but shows no mail
- Compare Exim’s local delivery format with Dovecot’s mbox or Maildir setting.
- Check mailbox ownership and permissions for the user account.
- Inspect Exim’s delivery log to determine whether the message was delivered, deferred or routed elsewhere.
ClamAV scanning is bypassed or every message fails
- Confirm that
clamdis running and that the socket path exactly matchesav_scanner = clamd:/var/run/clamav/clamd.ctl. - Check that the
clamavuser belongs to theDebian-eximgroup and that group membership took effect for the running processes. - Verify the DATA ACL condition that rejects
malware = *. - Restart
clamav-daemonand Exim after correcting permissions or configuration, then repeat the EICAR test.
Local delivery works but remote mail is rejected
Local protocol tests cannot prove reverse-DNS reputation, SPF, DKIM, DMARC correctness, provider-specific rate limits or acceptance by remote providers. Check the VPS provider’s SMTP policy, PTR record, sender-authentication records, certificate identity, queue and bounce message. If operating a smarthost, verify its credentials and account limits instead of assuming that direct-delivery diagnostics apply.
Operational choices that affect maintenance
Split versus single Exim configuration
Split configuration keeps routers, transports, ACLs and authentication fragments in separate files under /etc/exim4/conf.d. That structure is easier to review and change incrementally, especially when adding the ClamAV DATA ACL. A single template can reduce file navigation for a minimal installation. Whichever layout you choose, make source edits, run update-exim4.conf and restart deliberately so the running daemon matches the files you reviewed.
IMAP versus POP3 for users
IMAP keeps message state on the server and is generally the better fit for multiple devices, folders and synchronized read status. POP3 is a simpler retrieval model and can be appropriate for a single-device workflow, but users must understand download and deletion behavior. Dovecot can provide both; expose only the protocol ports you intend to support.
Direct delivery versus a smarthost
Direct delivery gives Exim control over remote SMTP conversations but makes you responsible for reputation, reverse DNS, TLS interoperability, queue management and block-list remediation. A smarthost reduces those responsibilities but introduces provider dependence and may impose recipient, volume or authentication limits. Neither route removes the need for local TLS, mailbox security and malware scanning.
Production checklist
- Supported Debian or Ubuntu release with a stable FQDN.
- Provider approval for required mail traffic.
- Forward DNS, MX and provider-managed reverse DNS planned and verified.
- Exim routing mode selected and generated configuration reviewed.
- Authenticated SMTP enabled and protected by TLS.
- Trusted certificate name matching the mail FQDN installed for Exim and Dovecot.
- Dovecot mailbox format aligned with Exim delivery.
- Only intended IMAP, POP3 and SMTP ports exposed.
- ClamAV signatures updating,
clamdreachable and the malware DATA ACL active. - Local POP3 or IMAP, authenticated SMTP and EICAR rejection tests recorded.
- Backups, disk monitoring, queue monitoring, certificate renewal and log review assigned to an operator.
Passing this checklist means the three services are wired together and behaving as configured. It still does not guarantee delivery to every external mailbox; that requires ongoing DNS, reputation and provider-response management.
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