Postfix - forwarding some email addresses to an alias through external SMTP service

Question

I'm trying to set up a Postfix server on Google Cloud, which blocks outgoing SMTP ports. My problem is that one email address (say [email protected]) needs to be an alias for an external email address ([email protected]); all emails sent to [email protected] need to be forwarded to [email protected]. I also have other email addresses that simply need to be delivered locally.

I am able to get Postfix to attempt to send all emails to [email protected] to [email protected] using virtual alias mappings, but because SMTP ports are blocked, the connection doesn't go through to example2.com's SMTP server. I want to get Postfix to send these emails through Mailjet on port 2525 so that the connection isn't blocked. Unfortunately, even when relayhost is set to in-v3.mailjet.com:2525, Postfix doesn't attempt to relay the emails and instead tries to send them directly.

I'm on Ubuntu 16.04.

Answer 1

When you do any relay directive, postfix by default will try to lookup the mx for that domain, so you have to tell postfix to use the a-record provided using brackets, see my relayhost directive below.

relayhost = [vps1.domain.com]:26

I'm using an off port to get around my ISP blocking smtp, so this forces postfix to go straight to that a-record and connect to send email.