We are building a new system that will support a large number of users (high
volume, high concurrent usage, etc).
what is large?
We have played with Dovecot, but in most serious applications we have
traditionally used Courier IMAP.
It's my (lay) understanding that with indexing and perhaps other things
in Dovecot, it might perform better than Courier in larger environments
like this. Am I correct or is it less clear-cut?
No idea how well courier IMAP performs. But have idea how well dovecot
performs. I don't have large configs like thousands of users as i don't
handle "herd of random users" style cases, but in every place i have
dovecot IMAP takes unnoticable amount of server load.
Just make a test.
Definitely use maildir format, not mbox.
dovecot heavily accesses it's index files. they are not large relative to
e-mail sizes. With really large case if I/O will limit you i would
recommend using SSD storage to keep just indexes.
