I use Amazon SES as a mail relay service for my emails as my server host blocks connections out on port 25. SES works great, however occasionally users send large attachments (>10MB). Whilst I could just every time suggest they upload their attachments somewhere and send a URL, this becomes tiresome and sometimes too complicated for my users. I have another mail relay available that allows me to send a smaller number of larger emails and so would like to send messages larger than 10MB through an alternate mail relay, and all other mail through Amazon SES still.
How would it be possible to achieve this in a postfix smtp server?