exchang server stops sends emails when an large email has been sent, and does not respond untill transport services has been restarted

Hi

Exchange server(CAS and mailbox) stops sending emails,  all messages are sitting in the users outbox during this period, Inbound emails however continues to work and the outbound does not start until transport services has been restarted. I have noticed that most of the time around that period one of the users would have sent a large attachment in an email ( <100MB checked at the email firewall ). however when we checked the outbound queue we don't see the large email.

We have recently migrated from Lotus to Outlook, we did not have this kind of issue when we were in lotus ie sending emails with large attachments

is there a way to avoid this kind of behavior

thanks in advance

Anand