Issue: When sending email via Microsoft Outlook I get the following error
Server error: '501 <mailto:user@domain.com>: "@" or "." expected after "mailto"'
Solution:
When sending emails using outlook you might receive the error above. It seems that when you copy & paste the email addresses into the To field, you may have entered an incorrect character in the email address instead of "@" and ".". This is frequently seen when copying and pasting email in a keyboard character set which may have replaced "@" or "." with an alternate character.
If the above does not describe your case and your To email is correctly formatted then please try the following solution recommended by Microsoft
If using IMAP right-click or Control-click the Sent Items folder in the Navigation Pane to the left and select Repair Message List. Optionally, you can go to Outlook menu --> Preferences... --> Accounts and delete your IMAP account, restart Outlook and then add your account. This should clear your cache of message but they will return from the server after syncing again.Similarly, if using Exchange you can right-click or Control-click the Sent Items folder in the Navigation pane to the left and select Folder Properties... Click the Empty button to delete the cache of messages and they will download again from the server. Or do this with the Exchange account header in the Navigation Pane to clear the entire account's cache.If you're using a POP account the you may have something invisible stuck in your Outbox. Launch Outlook with the Option key held down. This will launch the Database Utility application. Select the option to verify your database and repair if necessary.