So, you want to receive e-mail via IMAP (?) from one server, but send via SMTP via a different server. Is this right?
Then, it is not a problem. When setting up your account in Aquamail, you just choose "manual", and manually write information for the IMAP and SMTP (different) servers in there.
(BTW, IIRC, identities have a setting for alternative SMTP servers!)
However, you mentioned that you are receiving e-mail from an exchange server... If you are connecting to it via EWS (Exchange Web Services), - I believe, the situation is different. AFAIK, with EWS, you don't have a separate SMTP server. Not having first experience with the exchange server settings, - I cannot tell you if there is a workaround for your problem. Kostya (or someone else) might be able to help for that situation.