Outlook on MacBook sent email remain in Outbox but when switching to Legacy version emails sent fine

Hi @Jakub Gutowski

Based on the information you provided, I understand that your client's Outlook for Mac is failing to send emails, they get stuck in the Outbox while receiving works perfectly. The fact that "Legacy Outlook" works fine suggests this might be a specific synchronization or protocol issue within the New Outlook architecture.

To help me investigate this further and provide a more targeted solution, could you please provide the following details:

-What type of account is the client using? (e.g., Microsoft 365 Business, Exchange, Outlook.com, IMAP, or Gmail?)

-Could you provide the exact version of macOS and the Outlook build number? (You can find this in Outlook > About Outlook).

-When you reinstalled Office, did you download the installer from the Mac App Store or directly from the Microsoft website?

If you create a new macOS user profile and set up the account there, does the behavior persist? This helps determine if the issue is tied to the user's local library or the application itself.

Please let me know more details!

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In New Outlook for Mac this behavior is typically caused either by a connectivity/authentication problem to the SMTP server or by a corrupted/mis-mapped Sent Items/Archive folder in the mailbox. Legacy Outlook uses a different sync stack, so it can continue to send while New Outlook fails.

Use the following checks and fixes, focusing on the New Outlook profile only:

1. Verify Outlook is online and network is working

• In Outlook for Mac, confirm Outlook > Work Offline is not enabled.
• Use a browser to confirm general Internet access and that webmail for the same account can send successfully.

2. Check SMTP authentication and server settings For POP/IMAP or non-Exchange accounts, incorrect or blocked SMTP settings will leave messages in Outbox.

• Go to Tools > Accounts.
• Select the affected account.
• Under Outgoing server, click More Options and:
  • Ensure Authentication is enabled as required by the provider.
  • Re‑enter username and password if needed.
• If the ISP/network uses a firewall that blocks the current SMTP server:
  • Obtain an alternative SMTP server/port from the ISP or mail provider.
  • Enter that alternative server in Outgoing server.

3. Confirm account credentials and special requirements

• In Tools > Accounts, verify the correct email address, username, and password.
• If the provider requires SSL or non‑default ports, configure those exactly as specified by the provider.
• If the account still does not work, validate the full configuration with the email provider (including any required SSL/port overrides).

4. Check for proxy requirements on the Mac If the Mac is on a corporate network that requires a proxy:

• In macOS System Settings/Preferences > Network > (active network) > Proxies, ensure the mail proxy settings match what the network administrator provides.

5. Check if the Sent Items/Archive mapping is corrupted (Exchange/Microsoft 365) If this is an Exchange or Microsoft 365 mailbox and messages are stuck only in New Outlook, the default folder mapping (especially Sent Items) may be corrupted.

Use one of these methods to reset default folders (run against the same mailbox):

Method A – Outlook for Windows /resetfolders

1. On a Windows machine with the same mailbox configured, close Outlook.
2. Press Windows key + R.
3. Run:

      Outlook.exe /resetfolders
   

4. Restart Outlook and let it fully sync.

Method B – Outlook Web App language reset

1. Sign into the mailbox via Outlook Web App.
2. Go to Settings (gear) > Options > General > Region and time zone.
3. Change the language to another language, check Rename default folders so their names match the specified language, and Save.
4. Change back to the preferred language, again selecting Rename default folders, and Save.
5. Restart Outlook on the Mac and test sending again.

Method C – Admin: Set-MailboxRegionalConfiguration If this is an Exchange Online/on‑premises Exchange mailbox and admin access is available, run for the user:

Set-MailboxRegionalConfiguration -id <alias> -LocalizeDefaultFolderName:$true -Language <LanguageCode> -DateFormat <PreferredDateFormat>

Then restart Outlook for Mac and test.

6. If folder mapping is badly broken (advanced) If the Archive/Sent Items mapping is severely corrupted, use the advanced MFCMAPI method from a Windows Outlook profile to:

• Create a new Archive folder.
• Copy the ArchiveFolderEntryId from the current Archive folder to the new one.
• Run Outlook.exe /resetfolders and Outlook.exe /resetfoldernames. Then test again in New Outlook on Mac.

7. Recreate the profile only after the above Since the profile has already been recreated and the issue returns, focus on fixing server‑side folder mapping and SMTP/auth settings first. After applying the fixes above, if needed, remove and re‑add the account in New Outlook once more so it picks up the corrected server configuration.

References:

• I can't send or receive messages with Outlook for Mac
• Email messages are stuck in Outbox in Outlook 2016 for Mac
• Outlook email stuck