I’ve been reviewing MXGuarddog as a anti-spam solution, and on a cPanel server I set it up for a domain I manage and results have been awesome - so I was curious as the provide instructions to use some mailserver rules to ensure mail is coming from their filtered and not from someone ignoring my MX Records, would it be possible to limit inbound email connections to their servers?
They provide this document (https://mxguarddog.com/blog/prevent-direct-delivery/) any way to easily impliment that for Mailinabox?
If you are considering use of MXGuardDog (awesome little service) I’d reccomend the following in your /etc/postgrey/whitelist_clients:
## MX GUARD DOG
servers.ik2.com
122.103.250.12
85.195.83.79
64.38.239.80/29
## MX GUARD DOG /END ##
Then restart postgrey service (I find I have to kill it to get it to refresh, but YMMV). These IP’s and such are listed on their site, verify them before entering if you decide to add this to make sure you’ve got the latest and greatest.
I’m still working out the firewall rules, but may create more problem than good with mobile phones and sending email myself - odd port could work as they suggest, but for now this keeps mail flowing and spam cut down even more, if MXGuardDog doesn’t quarantine it, SA is at least given a chance to filter.
Nothing one I noted, you’ll need to update your SPF records which by default use the option mx to define what is allowed to send as you, you’ll need to add ip4: with the IP of your server to ensure the SPF remains valid since some spam firewalls are VERY particular about valid SPF, as I learned this morning when I hit a “doh!” moment 
Hope this helps someone in getting email filtering working smoothly, really just documenting my efforts should anyone else come and need to do the same.
Working on the firewall efforts this week, alt. port may be best, so I’ll post more on that final decided config once it’s done.