Hello - I have discovered what seems to be an Outlook for Mac 2011 bug:

1. Create two IMAP accounts on the same IMAP server but with different usernames and passwords.  For example, I have a personal Gmail account and a work Google Apps account.  These both use imap.gmail.com and smtp.gmail.com but have totally different usernames and passwords.

2. Send an e-mail from IMAP account #1 (let's say the work account).

3. Before the other e-mail has finished sending, send an e-mail from the other IMAP account, the personal one (#2).

4. Note in your sent folder that both went out from IMAP account #1.

The issue appears to be that Outlook checks if the server is the same and if so assumes that the credentials must be as well.

Two questions:

1. Has anyone else seen this problem?

2. Does anyone have a fix for this problem other than "send e-mail more slowly?"

Thanks!