[Dovecot] Excessive imap_login processes

I'm running Dovecot on a very small mail server (never more than six users accessing IMAP, often fewer). I'm seeing the number of imap_login processes grow over time, and right now there are 23. All the "login_process_*" settings in dovecot.conf are commented (ie, at their defaults).

Is this a known or expected issue? The version of Dovecot is 0.99.10.8, which I accept is perhaps a little old.

Thanks, Keith