IMAP
Of course, webmail wasn't working on the new box after setup. IMAP is run via inetd, so edit /etc/rc.conf and add:
inetd_enable="YES"
inetd_flags="-l -R 1024"
Make sure the line
imap4 stream tcp nowait root /usr/sbin/imapd imapd
in /etc/inetd.conf is uncommented.
Starting inetd myself:
/usr/sbin/inetd -l -R 1024
webmail was now working. Having done this so many times, I always forget..