In the E-Mail Name Resolution section, click Clear Most Recent Recipients list, and then click OK in the confirmation dialog box. While you're still in Options , to return your mailbox to the full version of Outlook on the web, go to Outlook version ,

If the recipient has such a rule, they'll need to correct the destination email address or remove the rule in order to prevent 5.1.x message delivery errors. Microsoft 365 and Office 365 support multiple ways to forward messages automatically.

As a first step in troubleshooting email delivery problems for Office 365 for business, we recommend that you download and run Microsoft Support and Recovery Assistant on the affected machine. Note that if you are experiencing issues with Outlook for Mac