If email seems to have disappeared after a Microsoft 365 migration, first find out whether Exchange Online received it. Then check the route the sender used, whether the intended recipient owns the address, and whether the user can access the mailbox where the message arrived. A missing message is a symptom—not proof that mail was permanently lost.
Start by locating where delivery failed
Before changing DNS, addresses, or permissions, identify what kind of migration took place—such as a staged, cross-tenant, hybrid, or IMAP migration—and define the scope. Is the problem limited to one address, affecting a whole domain, happening only with external senders, or occurring with internal mail too? Is mail not arriving at all, arriving somewhere unexpected, or present in a mailbox the user cannot open?
- Send two controlled test messages: one from an external address and one from an internal Microsoft 365 account. Record each sender, recipient, send time, and any non-delivery report (NDR) or bounce.
- Trace each message in Exchange Online: use message trace and mail-flow diagnostics to check whether Exchange Online received it and what happened next. Microsoft’s mail-flow troubleshooting guidance and mail-flow diagnostic explain the available checks.
- Use the result to choose the next check: if Exchange Online did not receive an external test, investigate routing; if it received the message, check the recipient and mailbox; if the issue is opening or sending from a shared mailbox, check access and hybrid configuration.
Do not treat an empty Outlook folder as evidence of non-delivery. The message trace, NDR, recipient address list, synchronization status, and permission state help distinguish the failure types.
| What you observe | Evidence that helps narrow it down | Where to investigate |
|---|---|---|
| External mail fails, but internal mail works | External test’s message trace or NDR; live MX destination | Public routing, gateways, and connectors |
| Mail to one address fails or goes to an unexpected mailbox | Recipient’s email addresses and any proxy-address conflict | Recipient object and address ownership |
| Only synced or hybrid recipients are affected | On-premises recipient attributes and directory synchronization errors | Source-of-authority and synchronization state |
| Mail reaches the shared mailbox, but a user cannot open or send from it | Mailbox existence, access permissions, and hybrid remote object | Shared-mailbox access path |
Why external email may still go to the old system
External senders use the domain’s public MX record to decide where to route mail. Compare the live MX record with the Exchange Online value shown in the tenant’s domain and DNS settings. Also confirm whether your intended design still routes mail through an on-premises server, gateway, or connector; a non-Microsoft 365 MX destination is not necessarily wrong if that route is deliberate.
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After a cutover, some sender systems may continue using a cached old MX destination until its time to live (TTL) expires. That differs from a live DNS record that still points to the old service: the first can affect some senders temporarily even after DNS has changed, while the second means the published route remains old. Microsoft’s staged-migration guidance recommends lowering the MX TTL before cutover to reduce delays and says a short TTL, such as 3,600 seconds (one hour) or less, can be used. This is preparation guidance, not a guarantee that every sender will switch immediately.
Do not make another MX change until you have confirmed the intended final mail-flow design. Microsoft recommends checking connectors and MX or SPF configuration when mail flow fails; its cross-tenant migration guidance is also relevant when the migration involved moving mailboxes between Microsoft 365 organizations.
Why an old address or alias may stop working
An alias, also called a proxy address, is an additional email address assigned to a mail-enabled recipient. It is not a separate mailbox: mail sent to that address is delivered to the recipient’s primary SMTP address. Microsoft describes this behavior in its mailbox address management guidance.
- Confirm the destination recipient exists. In the Exchange admin center or Exchange Online PowerShell, inspect the intended recipient’s type and email addresses. Check that the old address is listed on the mailbox or other intended recipient, not merely in a user’s Outlook contacts.
- Check for an address conflict. If Exchange will not accept the address, search mail-enabled recipients for the exact SMTP address. A proxy address cannot be assigned to multiple objects at once. Microsoft’s proxy-address conflict guidance covers this error.
- Verify the owner before removing or moving an address. A duplicate may belong to a different active recipient. Identify which object should receive mail before resolving the conflict, then test delivery to the address again.
If the address is listed on the correct object but messages still do not arrive, use the message trace and NDR to determine whether the failure is upstream of the recipient or later in delivery.
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What to check for directory-synced or hybrid recipients
For recipients synchronized from on-premises Active Directory or Exchange, compare the on-premises Exchange recipient attributes with the Exchange Online address list, and review Microsoft Entra synchronization status and errors. A cloud-side edit may not persist as expected when the on-premises object is the source of authority. Make corrections through the supported management path for that environment rather than applying a cloud or directory attribute edit as a universal fix.
Microsoft documents a case in which a UPN change can leave the MailNickname or Alias attribute inconsistent between on-premises Exchange and Exchange Online. Its MailNickname/Alias troubleshooting article recommends correcting the appropriate on-premises Exchange object so directory synchronization carries the change. For migration errors involving target SMTP proxies, compare the source and target proxy-address lists and verify synchronization using Microsoft’s target SMTP proxy migration guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why users cannot open or send from a shared mailbox
First confirm that the shared mailbox exists and that affected users have the permissions their task requires. Full Access lets a user open and manage the mailbox; sending as the mailbox requires Send As or Send on Behalf permission, depending on how the message should appear. Microsoft’s shared mailbox guidance explains mailbox setup and access.
- Cannot see or open it: check Full Access and make sure the user is opening the intended mailbox.
- Can open it but cannot send as it: check whether the user has the required Send As or Send on Behalf permission.
- Hybrid users cannot open it or resolve its SMTP address: check whether the on-premises Exchange organization has the corresponding remote shared-mailbox object.
In a hybrid environment, creating a shared mailbox directly in Exchange Online without a matching on-premises object can prevent hybrid users from opening it or resolving its SMTP address. For applicable supported environments, Microsoft’s hybrid shared-mailbox troubleshooting guidance documents creating a matching on-premises remote mailbox with New-RemoteMailbox and the -Shared switch. This is a hybrid remedy, not a step for a cloud-only tenant. Newly granted access can also take time to replicate.
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If the Microsoft 365 domain is newly added
When the affected domain was just added to Microsoft 365, confirm that it is verified in the Microsoft 365 portal and that its MX record matches the Exchange Online value shown for that domain. Microsoft’s new-domain troubleshooting article gives scenario-specific guidance of up to one hour for domain replication and up to 72 hours for DNS MX replication. These are not guaranteed wait times for every migration or DNS change.
If messages still fail after checking the domain and route, return to the test message’s trace or NDR and follow the evidence to the next failing point rather than changing DNS again.
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