Outlook Outbox will not send messages.

Send a small, attachment-free test message from Outlook on the web; if it sends there, rebuild the Microsoft 365 account connection in Outlook for Mac rather than restarting the Mac again.

1. Determine whether the Mac app or Microsoft 365 is affected

1. Sign in to Outlook on the web with the same business account.
2. Send a short message to yourself without an attachment.
3. Check the result:
   • Web message sends: the mailbox and Exchange Online can send, so continue with the Mac-client steps below.
   • Web message also fails: skip to If Outlook on the web cannot send.

2. Clear anything blocking the Outbox

Open Outbox and inspect the oldest message first.

• Delete it temporarily, or move it to Drafts.
• If it contains an attachment, remove the attachment and send a link to the file instead.
• Then create a new, attachment-free test message.

Microsoft identifies large attachments as a common reason messages remain in the Outlook for Mac Outbox.^1^

Also select the Outlook menu and verify Work Offline is not selected. Outlook for Mac cannot send while operating offline.^2^

3. Update Outlook

In Outlook, select:

Help → Check for Updates

Install any available update, reopen Outlook, and send another attachment-free test message. If Outlook displays error 3253 or “connection to the server failed or was dropped,” Microsoft specifically recommends updating Outlook and then using Help → Contact Support if it continues.^1^

4. Re-add the business account if webmail works

If Outlook on the web sends successfully but the Mac app still does not:

1. In Outlook, open Outlook → Settings → Accounts.
2. Select the affected Microsoft 365 account.
3. Remove the account.
4. Add it again using the business email address.
5. Allow Outlook to finish resynchronizing before testing.

Removing the account deletes locally cached Outlook data, so confirm that any unsynchronized local-only drafts or folders are preserved first. Re-adding the account is the documented next client-side step for the matching situation where Outlook for Mac receives mail but leaves sent mail in the Outbox.^3^

Do not manually change SMTP settings unless this is actually a POP or IMAP account. SMTP authentication and outgoing-server settings apply conditionally to those account types, not normally to a Microsoft 365 Exchange account.^2^

If Outlook on the web cannot send

Ask the Microsoft 365 administrator to check:

Microsoft 365 admin center → Health → Service health → Exchange Online

The administrator should review both Active issues Microsoft is working on and Issues for your organization to act on. If no matching incident appears, they can select Report an issue from Service health.^4^

Also check mailbox storage. Microsoft states that a full Microsoft cloud mailbox can prevent sending and receiving, although continuing to receive messages makes that less likely here.^2^

If the problem remains, use Outlook → Help → Contact Support and include:

• Outlook version from Outlook → About Outlook
• macOS version
• Whether Outlook on the web can send
• Any exact error code or message
• Whether a new message without attachments also remains in Outbox

Microsoft currently lists some Outlook for Mac send and synchronization problems on its known-issues page, so the exact error text is important for matching the applicable issue.^5^

References

1. Outlook for Mac Help | Microsoft Support
2. I can't send or receive messages with Outlook for Mac
3. Outlook app for Mac sending emails stay in outbox - Microsoft Q&A
4. How to check Microsoft 365 service health
5. Fixes or workarounds for recent issues in Outlook for Mac