Postfix forwards to Gmail being rate limited even with unmodified headers

I've installed Postfix on Ubuntu 14.04 with the defaults. I then configured some virtual aliases to forward email from a few addresses to various personal (mostly Gmail) accounts. This is just to make these addresses all connect cosmetically to the domain on the website we are hosting.

Unfortunately, in a short amount of time Gmail has started rate blocking the forwarded emails due to spam. Apparently, Postfix is rewriting the headers in a way that makes Gmail think all the spam is originating with our server. Google refers to this article for information on how to best forward to them. In it, they say:

We recommend that you do not change the envelope sender when forwarding email to Gmail. Sometimes, when forwarding email, the envelope sender gets changed to your domain. When this happens, Gmail may learn that your domain is sending spam, and will treat other emails from this domain as spam as well.

This make sense, but it would still seem that Gmail would have to see the IP and hostname of our mail server and register it as a source of spam. But this is what they say and I've found nothing on point anywhere.

I even tried using Mailgun to filter the forwards as they provide spam blocking, but within a day they deactivated the domain and are being rather slow to respond to my inquiries. Any help is appreciated. I would like to avoid a full fledged mail server setup just to handle a few rarely used forwards.

Update: My original assumption was incorrect in that Postfix is not rewriting the headers, however the original issue remains. Gmail is rate limiting forwards due to excessive spam. It would be understandable if they limited the servers that are actually sending the email, and these are listed in the headers as part of the delivery chain.

_{Short answer}

Do not forward spam. Reject it or Drop it before forwarding.

_{Long answer}

GMail hates spam as much as you do. So Gmail began to rate limiting your server because your server who throwing the spam upon them.

The article that you refer to was talking about domain not IP address. Both was different factors to consider when comes to email reputation.

• Sender domain: This factor is easily forged. However when the domain was valid and sending spam then the domain came into blacklist.
• Client IP address: This factor was considered because this server who send forward spam to us.

Just because you don't become source of spam, it doesn't mean you may forward spam. Spam should be terminated in the first place. Forwarding spam to other server (without filtering it) was looks like putting your responsible to other.

You can always be part of spam solution. See our canonical QA about fighting spam: Fighting Spam - What can I do as an: Email Administrator, Domain Owner, or User?