Hello everyone,

This will be a bit hard to explain but i have this kind of setup:
1. ScrolloutF1 SPAM Filter server, let's call it spam.lan
2. Original Mail server. Let's call this one mail.lan

Both of them on same local network but both of them have different public IPs.

spam.lan is set to be the MX for lan zone. It will receive messages, filter them, and if message is tagged as spam, process them accordingly and if not, it will forward the mail to mail.lan via local/private address.

mail.lan is used for webmail but it also has a imap/pop3 and smtp server on it, which clients can use to setup their mail accounts in outlook or similar email client, so they cen retrieve and send messages from their clients instead of using webmail.

My goal is:
I would somehow like to get mail.lan completely off the public network, remove it's public IP address and leave only private IP and i would like for spam.lan to act as mentioned imap/pop3 or smtp server. When it receives request from client, it should contact the mail.lan via private IP since they are in the same private subnet, pull the emails and forward them to users mail client.

I'm really stuck and i cannot figure out the way to do this.

mail.lan is a iRedMail server on CentOS 7. It stores users in mysql database and messages in /var/vmail dir.
spam.lan is ScrolloutF1 Server working on Debian 9. It has it's own dovecot and postfix but it's using them to relay received messages to mail.lan

Can i actually pull this off somehow?

Many years ago when I was working at a university, we had similar setup. Had a load-balancer (since all students and faculty used this); load balancer had public and private IP. It forwarded emails to AV-AS servers which then sent emails to email server. Communication between load-balancer <-> AV-AS server <-> email server happened on private network. AV-AS servers and email server ran SLES and load-balancer was an appliance. So, yes what you are trying to do is totatly doable.
Email service was moved to cloud provider, so no longer have documentation but if you have specific question it may jolt my memory.

Well that's the thing. I don't even know where to start configuring this or how am i supposed to do this. How to make it happen.
I already have server that is acting as antivirus and antispam. And an email server to which it forwards mails if they are clean, via private IP. But how to remove mail server off the public network so AV-AS server can be used for relaying.

And also would it be possible that AV-AS server be used as imap/smtp, for let's say, users configure it's hostname in Microsoft Outlook, and then normally receive and send email because that server will relay traffic to clients that it will 'pull' from private network via real mail server.

Tell me if I'm not making any sense since i am very poor when it comes to explanation