In case this helps anyone we've started using a $30 a year backup MX service with our Astaro's

We have add great luck configuring the MX records as follows

domain.tld. 2466 IN MX 10 mail.domain.tld.
domain.tld. 2466 IN MX 20 mail2.domain.tld.
domain.tld. 2466 IN MX 30 mx.domainofserviceprovider.tld.
domain.tld. 2466 IN MX 40 mail3.tdomain.tld.

Where mail2 and mail3 are A records pointing to the same IP as mail.domain.tld. which is our AMG2000

We had to turn on Blacklisting at the backupservice; but they offerred most of the same ones we run on the astaro.

of our 1.3Million spam per month only about 12k are coming in through the backup MX service.

That's a nice balance of uptime insurance without letting in hoards of spam. Hopely this helps someone else.

If you need to know the name of the service ask me in private messages I don't work for them but I don't believe in advertising for them on the astaro board. I'm sure this would work with any service.