I've added a second webmin server as a backup MX, checking "Add all existing mail domains to secondary MX servers?" when adding the server.
Testing secondary MX server x.x.x.x ..
.. done
Updating virtual server domain.co.uk ..
Adding secondary domain on x.x.x.x ..
.. done
Updating secondary MX records ..
.. done
.. done
However when i validate mail for my domains i get the following error.
Mail for domain : Secondary MX server x.x.x.x is not accepting email for this domain
The domains have not been added to relay_domains on the backup. If i create a new domain after enabling the backup MX then they do get added.
If i remove the backup MX webmin gives me the following error
Updating virtual server domain.co.uk ..
Removing secondary domain on x.x.x.x ..
.. failed : This server is not a relay for the mail domain
Updating secondary MX records ..
.. done
.. done
However the MX record is not removed