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