Understood.
We have a similar situation and have been thinking about the correct approach.
The best way, in my opinion, is if EIM matches the Exchange setting. When a customer sends in an email larger than the limit Exchange has (global) they get advised that they have exceeded the ability of that email system. If EIM cannot take the same sized email, they are, as you say left on the server - in the mailbox.
I did not know about the notification bit.
On the one hand, if EIM's limit matches the Exchange limit (at this customer it is 10MB) then EIM's file storage is going to fill up faster, mainly due to spam - the big emails customers get are often spam.
On the other hand, a really big multi-page fax order for a lot of product better come through or the customer will me much annoyed. $$$ are involved.
Regards,
Geoff