Exchange 2010 won't allow large attachments

I’m running Exchange 2010 v14.01.0438.000, having migrated from Exchange 2003. (If somebody knows FOR SURE that applying SP2 will solve this problem, I’ll do it ASAP. I just haven’t had time to go beyond v14.01.0438.000.)

The 2010 server is handling inbound and outbound SMTP traffic, hosts the Public IS and all of the user mailboxes, and is the only operational Exchange server. (The 2003 server is still online, but twiddling its thumbs.)

My problem: large attachments are blocked – senders are seeing attachments of over 15MB bouncing. (I haven’t tried smaller than 15MB.)

I’m running Symantec Mail Security (SMSMSE), but have effectively ruled that out as a problem as I tested inbound with the following services stopped: Symantec Mail Security for Microsoft Exchange, Symantec Mail Security Management Service, and Symantec Mail Security Utility Service. With these services stopped my test email of 23MB continued to bounce.

The firewall is not the issue, as I was receiving 20MB-plus attachments using the old mail-server (going through the same firewall with the same config).

My Exchange server is configured per the attached. The results from a typical bounce is also attached.

Any ideas or suggestions would be greatly appreciated.

THANKS LOTS!!!!
2010-Config.txt
Bounce.txt