If email is missing after a Microsoft 365 migration, first find out whether Exchange Online received it. A message may still be going to an old mail system, may have been rejected because an address is missing or duplicated, or may be in a mailbox the recipient cannot open. Test both external and internal delivery, then use the evidence to choose the right fix. The next steps depend on whether the migration was hybrid, cross-tenant, staged, IMAP, or another type.
Start by locating where delivery failed
Do not treat an empty Outlook view as proof that a message was lost. Send one controlled test from an external address and another from an internal account. For each, record the sender, recipient, time, and any non-delivery report (NDR). Use Exchange Online message trace and Microsoft’s mail-flow diagnostic to determine whether Exchange Online received the message. Microsoft’s mail-flow troubleshooting guidance also recommends checking connectors and MX or SPF configuration when mail flow fails.
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| What the tests show | Where to investigate first |
|---|---|
| External tests fail, but internal tests arrive | Public mail routing, including the domain’s MX record and any gateway or connector still in the intended route. |
| Both external and internal tests fail for one address | Whether the recipient exists and has the expected address; then check for an address conflict or synchronization issue. |
| Several recipients or the whole domain are affected | Domain verification, mail-flow configuration, DNS, and the migration’s routing design. |
| Trace shows delivery, but the user cannot see the message | The destination mailbox, the address that owns the alias, and the user’s access to the mailbox. |
| Users cannot open a shared mailbox or send from it | Mailbox existence, permissions, and—if the organization is hybrid—the corresponding on-premises Exchange object. |
Check MX routing before changing DNS
MX records tell external mail systems where to route messages for a domain. Compare the domain’s live MX record with the Microsoft 365 value shown in the tenant’s domain and DNS settings. Confirm whether the planned route also includes a mail gateway, on-premises server, or connector; an MX record that differs from Microsoft 365 is not necessarily wrong if that route is intentional.
After a cutover, some senders may continue using a previously cached MX destination until its time to live (TTL) expires. That is different from a live MX record that is still configured for the old destination. Microsoft’s staged migration guidance recommends reducing the MX TTL before cutover to reduce delay, and gives 3,600 seconds (one hour) or less as an example of a short TTL. Microsoft’s cross-tenant migration guidance also describes migration planning. Before editing DNS, confirm the final routing design and whether the migration is complete; changing MX prematurely can send new mail to the wrong system.
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Verify the recipient and the alias
In the Exchange admin center or Exchange Online PowerShell, check that the intended recipient exists, confirm its recipient type, and inspect its email addresses. An alias (also called a proxy address) is an additional address on a recipient, not a separate mailbox. Microsoft explains that mail sent to a proxy address is delivered to that recipient’s primary SMTP address in its mailbox address management guidance.
If Exchange reports that an address is already in use when you try to add it, search mail-enabled recipients for the exact address. A proxy address can belong to only one mail-enabled object at a time. Identify which recipient should own it before removing or changing anything; Microsoft documents the conflict check and resolution considerations in its proxy-address conflict troubleshooting article.
For synced or hybrid recipients, check the source of authority
When a recipient is directory-synced, compare its on-premises Exchange attributes, Microsoft Entra synchronization status or errors, and its address list in Exchange Online. A cloud-side edit may not be authoritative or persist as expected when the object is mastered on-premises. Make the correction through the supported source-of-authority path for that tenant rather than treating a direct Entra ID or Active Directory edit as a universal fix.
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Microsoft documents a specific UPN-change case in which the MailNickname (Alias) value in Exchange Online may not match the on-premises setting, and describes correcting the appropriate on-premises Exchange object so synchronization carries the change. For migration errors involving target SMTP proxies, compare the source and target proxy lists and verify synchronization. See Microsoft’s MailNickname or Alias mismatch guidance and target SMTP proxy migration troubleshooting.
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Check shared-mailbox existence, permissions, and hybrid representation
First confirm that the shared mailbox exists and that users are opening the intended mailbox. Check whether each user has Full Access and the appropriate Send As or Send on Behalf permission for the task they are trying to perform. Microsoft’s shared mailbox guidance explains these access types. Newly granted access may take time to replicate.
In a hybrid organization, also check whether the on-premises Exchange organization has the corresponding remote shared-mailbox object. Microsoft documents a case where creating a shared mailbox directly in Exchange Online without an on-premises object can prevent hybrid users from opening it or resolving its SMTP address. For applicable supported environments, Microsoft’s documented remedy is a matching on-premises New-RemoteMailbox with -Shared. This hybrid-specific remedy does not apply to a cloud-only tenant; consult Microsoft’s hybrid shared-mailbox troubleshooting guidance before making changes.
If the domain is new, verify its status and MX value
For mail problems affecting a newly added domain, verify the domain in the Microsoft 365 portal and confirm that its MX record matches the value listed for Exchange Online. In its new-domain troubleshooting guidance, Microsoft gives up to one hour for domain replication and up to 72 hours for DNS MX replication in that scenario. These are scenario-specific estimates, not a guaranteed wait time for every migration or DNS change.
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- An NDR identifies an address or recipient problem: verify that recipient and its address list, then investigate a duplicate proxy address if Exchange reports a conflict.
- Exchange Online did not receive the external test: inspect the public MX route, any intentional gateway or connector, and whether the sender may still be using cached DNS.
- Exchange Online received the message, but it is not visible to the expected user: confirm which recipient owns the destination address and whether the user can access that mailbox.
- The issue concerns a synced recipient or a hybrid shared mailbox: compare cloud and on-premises state and follow the supported hybrid or synchronization path for the specific object.
Microsoft’s migration overview outlines different approaches to moving multiple email accounts; the correct repair depends on the migration method and the trace, NDR, recipient, and permission evidence in your environment.
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