Unable to send email attachments > 3mb

We run Exchange2007 here in the Uk, our sister compay have there mailboxes located on our mail server, they use Office2003 SP3 to connect over a VPN (connection is x4 T1 US side and 2mb UK side).
The pipe has never been issue before, and the problem only affects Outlook2003 users (Outlook 2007 users are fine). Sitting either end of the pipes are Sonic Pro 2040 firewalls providing the VPN and Exinda packet shapign devices. Recently both firewalls and Exinda firmware levels were upgraded, this is the only change we can guess has caused the problems, MTU settings on the firewall and exinda are 1500

now when a user in the US attempts to send or receive a email over say 3mb , outlook just loops attempting to send the email with the attachment , i have included acreens shots of the users desktop. As i said Outlook 2007 users are not affected , we have turned on the Oulook logging, this is included in the screen shot but doesn't help?
Manually copied large files across the VPN are fine, other connections such as internet and citrix run fine,  Could it be a sonic wall VPN issue ?    
Outlook-error.doc