One of my user's keeps losing Besadmin SendAs rights, and he is not a Domain Admin. I've checked every group that he is in and there's nothing I can see that should be applying the adminsdholder reset. Any ideas? He's the only person this is happening to and it only started a couple of weeks ago.

If the user is part of any AdminSDHolder group they will lose the send as permission.

Here is the complete list of AdminSDHolder Groups:
• Enterprise Admins
• Schema Admins
• Domain Admins
• Administrators
• Domain Controllers
• Cert Publishers
• Backup Operators
• Replicator Server Operators
• Account Operators
• Print Operators

If he is part of any of the ones above the best way to fix it is to a) remove the user from the group or b) run the dsacls script from The "Send As" right is removed from a user object after you configure the "Send As" right in the Active Directory Users and Computers snap-in in Exchange Server