Howto/20230921 Use a cheap VPS to bypass ISP blocking of port 25 to postfix mail server

Disclaimer

My Howtos are mainly for me to document the steps required in case I ever have to re-do them again. Hopefully they will help other system admins out there, but I regret that this is not my primary objective.

Problem

We just switched to a new Internet Service Provider (ISP) that blocks port 25, both ways, making us no longer able to send or receive emails.

Solution

• For outbound emails, use the ISP's smtp server to send emails.
• For inbound emails, use a cheap VPS (mail2.interpause.com) whose port 25 is open to receive and then forward emails to the main mail server (mail1.interpause.com) on external port 2025 and internal port 25 (not blocked by ISP). We already have a cheap VPS for our OpenVPN server with racknerd that costs 11USD/year so, for us, there is no extra cost to setup this mail forwarder. Our solution is based on the following guide to setup a backup MX: https://www.howtoforge.com/postfix_backup_mx

Steps

1. Setup MX records on domain provider to point to VPS (mail2.interpause.com)
   • For namecheap, it looks like this: