The homepage for vm-pop3d can be found
here.
vm-pop3d is a POP only mail-server and was the pop server of choice for early versions of DirectAdmin because at that time DirectAdmin didn't support IMAP.
I don't know if DA staff added IMAP on my request or not, but I do know that I wrote them (years ago) and explained that I really liked DA but I couldn't use it unless it also offered IMAP because my clients were already using IMAP on their Cobalt RaQs and on their Plesk-based accounts.
Soon afterward, DirectAdmin started also offering the WU-IMAP daemon so we could also offer IMAP accounts.
But one of both of those daemons (I don't remember which) would only work with the
mbox mail format. And many of us, experienced with the
Maildir format, asked for support for that in DirectAdmin.
Since it wasn't possible to directly support Maildir for DirectAdmin with the default pop and imap servers, vm-pop3d and WU-IMAP, DA staff created an optional Dovecot installation.
Dovecot (homepage
here) is now what many of us use.
Jeff