I am having same problem..

All of a sudden outlook send an error message from system administrator saying user not found. This comes back in microsec so it never rteally left outlook.
Win 7 Outlook 2010 ISP Comcast.
Have another email on same server that works OK. no problem.

Have tried for 3 weeeks. Restore to factory setting, installed outlook, copied PST from before restore.

Tried both inputting from old pst and generated a new pst but same results.
Same results.

Checked all POP SMTP settings between the two they are identical.

Help