Due to regulations we have to create a solution that communicates over smtp/pop3 (basically RPC over SMTP/POP3) only.
Messages are sent from one application with SMTP, delivered to a mailbox, and picked up by another application with POP3, whereafter it delivers an answer with SMTP, and this goes on in a circle.
Estimated average message size is 40kB and estimated daily message count is 12-15 millions. This will be distributed to a small amount of user accounts, aproximately 10. No mail should be stored for a long time, it's deliver, pickup, delete.
We plan on using hardware with fusionio storage, and postfix for the SMTP part but are looking for a, preferably, open source POP3 software that can handle these volumes.
I've been looking at Dovecot, and it might be the best solution, but I'm worried that due to it's primarily focus on IMAP has some performance disadvantages for POP3.