Just tested Pop3 mail access to a a recently installed Virtualmin box (on Ubuntu) and it didn't work. (IMAP works fine.) The client appears to connect fine but doesn't download any messages. I looked in /var/log/mail.err and found lines like this:
dovecot: pop3(joeuser): pop3_uidl_format setting is missing from config file
dovecot: child 28242 (pop3) returned error 89
/etc/dovecot/dovecot.conf has this setting commented out with this descriptive text:
# POP3 UIDL format to use. You can use following variables:
#
# %v - Mailbox UIDVALIDITY
# %u - Mail UID
# %m - MD5 sum of the mailbox headers in hex (mbox only)
# %f - filename (maildir only)
#
# If you want UIDL compatibility with other POP3 servers, use:
# UW's ipop3d : %08Xv%08Xu
# Courier version 0 : %f
# Courier version 1 : %u
# Courier version 2 : %v-%u
# Cyrus (<= 2.1.3) : %u
# Cyrus (>= 2.1.4) : %v.%u
# Older Dovecots : %v.%u
#
# Note that Outlook 2003 seems to have problems with %v.%u format which was
# Dovecot's default, so if you're building a new server it would be a good
# idea to change this. %08Xu%08Xv should be pretty fail-safe.
#
# NOTE: Nowadays this is required to be set explicitly, since the old
# default was bad but it couldn't be changed without breaking existing
# installations. %08Xu%08Xv will be the new default, so use it for new
# installations.
#
#pop3_uidl_format =
Setting it to %08Xu%08Xv as suggested and restarting dovecot allows POP3 to work as expected.