You must map the DOMAIN to a PRIVATE, STATIC IP ADDRESS in the EDIT DOMAIN TAB
You must tell SMTP OUT to use the DOMAIN'S IP ADDRESS.
You MUST have your PROVIDER map an rDNS record for the PUBLIC IP ADDRESS to the DOMAIN to which it is assigned.
They you must MAP the PUBLIC IP ADDRESS used for the domain to a PRIVATE, STATIC, INTERNAL IP ADDRESS in SmarterMail and use ONLY THAT PRIVATE IP ADDRESS for the domain.
You must also create a HOST MAPPING, using the PUBLIC UP ADDRESS in the SmarterMail mappings.
To the best of my knowledge, you cannot setup priority on IP addresses in SmarterMail.
There is no interaction - just ONE-ON-ONE MAPPING, on a PER DOMAIN BASIS.
You will use the PUBLIC, STATIC IP ADDRESSES for your HOST ("A") records - pointing to your mail host.
You will use the same PUBLIC, STATIC IP ADDRESS for the rDNS for the domain's MX SERVER rDNS mapping.
You will create a MAIL RECORD and point it to the FULLY QUALIFIED HOST NAME for the domain.
This requires a very solid knowledge of both DNS and IP ADDRESSING.
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