Should I make use of the Backup Mail Exchanger (aka backupmx) feature, and will it increase the amount of spam I receive?
We recommend that you do not use the Backup Mail Exchanger ( backupmx ) feature due to the increased risk of receiving large amounts of unfiltered spam, unfiltered malware, and other types of illegitimate emails.
When a backup mail exchanger is in use, all mail that is sent through the backup mail exchanger skips most of the validation, malware, and spam checks that would normally occur if the message were received on the primary server. This is by design.
While this feature does offer a secondary mail host to accept messages in a situation where the main mail host is offline, it enables spammers and attackers to send messages directly to the Backup MX server to bypass most checks and deliver an illegitimate message directly to the inbox of the email accounts on the server. There is not currently any feature that is able to mitigate the increased illegitimate messages when using the Backup Mail Exchanger feature.
If you would like to have a feature added to address this concern, please file a feature request: