OK, I got this weird issues happening to my CFO. If it wasn't my CFO, I'd say too bad, but...
So the CFO uses the scheduling assisntant in Outlook. He begins to type a specific users name (it only seems to happen with this one user), then hits ctrl+k to resolve the address. This user is an internal user to my org.
Now, whenever he sends a meeting request, he recieves an NDR saying that the email address is formatted incorrectly - and it is, it comes in as "user%20domain.com" where user is the correct user name, and "domain.com" is our email domain. If I do the same, it seems to resolve the "@" sign properly. From what I can tell, he is the only user with this problem.
A few notes as well, the CFO is a laptop/mobile user who is in cached mode. As far as I can tell this came about when we migrated from Exchange 2003 to 2010 a few weeks ago.