Server replied: 421 Unexpected failure, please try later

If Imap returns this error, then there might be something wrong with the smtp server. Check the /var/log/exim/mainlog (or /var/log/exim/exim_mainlog).

Errors seen before:

failed to open /etc/virtual/pophosts for linear search: No such file or directory

Fix:

touch /etc/virtual/pophosts

That would also imply that da-popb4smtp isn't running, so popb4smtp wouldn't work. To start it, run:

service da-popb4smtp start
or FreeBSD:

/usr/local/etc/rc.d/da-popb4smtp start