I have two Ubuntu 14.04LTS servers running latest Plesk Onyx 17.0.17 Update #12
first server is primary MX server, second server I have configured
Postfix to act as backup MX server in order to have e-mail reliability and redundancy, when primary MX is down
however, it is NOT possible in Plesk Onyx to WHITELIST IP address of backup MX server
you can whitelist domain or e-mail address, but that means you would have to whitelist every e-mail address on planet earth (doesn't make sense)
Plesk NEEDS to add option to WHITELIST IP ADDRESS in order to exclude it from SPF and Greylisting, but leave DKIM, AntiVirus, and SpamAssassin checks in place
relaying breaks SPF so this is the reason why we need exception (exclude backup MX IP from SPF checks)
Greylisting own backup MX on primary MX server doesn't make sense either, only causes delay in delivery, so we need option to whitelist backup MX IP for greylisting
long story short,
it is not possible to use Plesk Onyx together with external backup MX due to Plesk SPF and Greylisting lack of functionality
1. there is no whitelisting for SPF whatsoever
due to rubbish "postfix-policyd-spf-perl" it has NO config file
according to
https://www.lux-medien.com/blog/policyd-spf-and-whitelisting.html "postfix-policyd-spf-python" does the trick - and it supports easy whitelisting
2. there is only domain whitelisting for Greylist
so you would have to whitelist every e-mail address and every domain *@* in order to allow getting e-mail from BackupMX without delay (which effectively kills greylist functionality)
Cmon Plesk Team, do the basic math and fix this!