failed to open DB file /var/spool/exim/db/wait-remote_smtp: Permission denied
The problem:
# ll /var/spool/exim/db/wait-remote_smtp
-rw-r----- 1 root root 12288 Mar 26 06:00 /var/spool/exim/db/wait-remote_smtp
Easy fix:
# chown mailnull.mail /var/spool/exim/db/wait-remote_smtp
# ll /var/spool/exim/db/wait-remote_smtp
-rw-r----- 1 mailnull mail 12288 Mar 26 06:00 /var/spool/exim/db/wait-remote_smtp