Linux – Postfix smtp relay + original sender address

As far as I know, if you use Gmail's SMTP as a relay it rewrites the From address to the Gmail account. Also, any reply-to address is rewritten to the same Gmail account. They've done this for years.

I've run into the same thing when I tried setting Gmail as my relayhost in Postfix. This does not happen with other relays I've tried, so I believe it to be a Gmail thing. I assume Google does this to prevent systems marking the mail as spam since the original From cannot be verified easily.

For individual non-gmail adresses you could configure Gmail by verifying the address. See: http://mail.google.com/support/bin/answer.py?answer=22370

For all adresses in a domain you could configure the domain to use Google Apps, although I've not tried this. Unfortunately this won't help if you relay mail for a number of domains.

See this question for more discussion on this subject: SMTP relay through GMail overrides "from:" address with "[me]@gmail.com"

Top tip: just don't do this. It won't take long, then you'll have this situation:

• Spammer sends spam to user1@mydomain.com
• You forward it to user1@gmail.com
• GMail sees spam from you and adds you to their list of people who send spam
• You don't get to send any mail to GMail any more until you change what you're doing

I've been there and done that. In fairness, there's no way they can tell for sure that you're not the person actually originating the spam. There'll be a bunch of extra Received headers in the mail, but you could fake those.

The other thing: SPF checks done by GMail will fail. Someone sends mail from (say) serverfault.com, you forward it to GMail, GMail checks serverfault.com's SPF record to see if your mail server is allowed to send mail for that domain, finds out it's not and drops your mail on the floor.

The correct way to do this is to keep the mail locally and have GMail pick it up via POP. GMail then knows that you're not the original sender, doesn't apply its spam checks in the same way and doesn't have the same problem with SPF.