TLDR; what's the correct way to config a backup mail service in SmarterMail?
We use an external backup mail service in case our local mail server ever goes down for maintenance or internet troubles etc. At one point early on, I noticed that legitimate messages routed thru those servers were being rejected (I can't recall why), so I added the backup MX server IPs to the Antispam -> Bypass Gateways page. But now I'm seeing that messages will get greylisted by Smartermail, then immediately get sent to the backup MX server and immediately delivered.. in fact, the bulk of our incoming mail is coming from those backup mail servers.
I'm wondering what is the correct way to configure this setup? Where should I place those MX IPs?