Conditional relayhost based on message size

[Alexander]

Hello,

My goal is to conditionally select the relayhost based on the total size 
of the outgoing message. The rationale is that I'm using Amazon AWS SES 
for the most part. Alas, SES only accepts messages up to 10 MB in size 
(this includes images and attachments that are part of the message), and 
when the outgoing message exceeds that size, it will be bounced.

Any tips for filters or tools that would help me define another 
relayhost based on the outgoing message size would be greatly 
appreciated!

Alex