This is really easy. First off you should have a backup of your
configuration.nix file where you have the server config (but that is already
in a git repository right?)
Next you need to backup /var/vmail or whatever you have specified for the
option mailDirectory. This is where all the mails reside. Good options are a
cron job with rsync or scp. But really anything works, as it is simply a
folder with plenty of files in it. If your backup solution does not preserve the
owner of the files don't forget to chown them to virtualMail:virtualMail if you copy
them back (or whatever you specified as vmailUserName, and vmailGoupName).
Finally you can (optionally) make a backup of /var/dkim (or whatever you
specified as dkimKeyDirectory). If you should lose those don't worry, new ones
will be created on the fly. But you will need to repeat step B)5 and correct
all the dkim keys.