I wonder if this is a clue to what may be going on. When I go to Mail - Accounts - and click on one of the email accounts I have, then click on "advanced" instead of going to the advanced page I get this message with each account I try: Advanced view With the exception that I don't have a Mail.com account I get the same thing you do. All that tells you is the URL for the IMAP server you are suing for that particular email account and should have nothing to do with you your messages are displayed on your iMac.
To expand upon my previous statement:
- On your computer messages are stored as records in a database (like images in Photos and tunes in Music)
- Those records/messages are accessed via pointers in an index file
- When you rebuild a mailbox Mail reads the records in the database and creates a new index of the database contents
- It is possible that the old index has been damaged and may show a message record that is no longer present or may have lost a pointer to a message record that is in the database.
- As a result the new index may have fewer entries, more entries, or the same number of entries as the previous version but it does not change the actual content of the database so messages are neither gained or lost in the rebuild itself. Rather the new index correctly reflects reflects the actual database/mailbox contents.
