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Single SMTP multihomed server behind Firewall

Configure SPAM Filter to listen on a different IP than your SMTP server. This can be done by assigning additional IP address to the server.Reconfigure the firewall so that it routes the external IP address to the IP assigned to SPAM Filter.The SPAM Filter gateway will now be receiving all internet email and will then forward legitimate emails to your SMTP server.

Single SMTP server / multiple IPs behind firewall DNS MX record: mail.domain.com · Firewall NATs to 10.10.10.2. After: DNS MX record: mail.domain.com · Firewall NATs to SpamFilter on 10.10.10.3. One server has two IP addresses; both services listen on port 25. 10.10.10.3: SpamFilter → 10.10.10.2: your SMTP server. Clients keep 10.10.10.2. BEFORE DNS MX record: mail.domain.com · Firewall NATs to 10.10.10.2 AFTER · WITH SPAMFILTER DNS MX record: mail.domain.com · Firewall NATs to SpamFilter on 10.10.10.3 Internet Incoming SMTP Firewall External SMTP port 25 Your mail server 10.10.10.2 port 25 Email clients SMTP: 10.10.10.2 port 25 SMTP ONE PHYSICAL SERVER Internet SMTP: port 25 Firewall NAT → 10.10.10.3 SpamFilter 10.10.10.3:25 SMTP server 10.10.10.2:25 Email clients 10.10.10.2:25 One server has two IP addresses; both services listen on port 25. 10.10.10.3: SpamFilter → 10.10.10.2: your SMTP server. Clients keep 10.10.10.2. Incoming mail → SpamFilter → Your SMTP server