And another Outlook problem that came up the other day...
I had migrated an SBS 2003 Server to an SBS 2011 Server. Note that the old server was completely broken and unusable for the normal migration path.
However some uses were unable to send mails to other internal recipients.
The solution for me, was:
1) Create a new mail
2) Start typing the address of the recipient, until you see the address in the autocompletion list field.
3) Move with the up/down keys to the address in question and hit 'Delete' on the keyboard.
Do this for every address you have a problem with (I only had two).
After that you have to type in the complete address for those recipients once, before autocompletion works again for those.