Hello Charles,
The index files are my main concern.
Well, that and having two different servers reading from potentially the same files/maildirs at the same time. Honestly, I just don't know enough about IMAP to know if there is anything to worry about or not... As far as I know, Maildir is implicitly locking safe and Dovecot handles it's index files very robust. If Postfix delivers a mail without using Dovecots deliver program, Dovecot also has to update its index files, so it shouldn't be a problem if Courier moves some mail around. Also it would only be a fair test if your tester is testing the servers one after the other and not simultaneously - you should tell him. So I don't
Charles Marcus wrote: think anything will get messed up, but you nevertheless don't want this type of setup for anything else than testing ;)
Regards, Philipp