This article is for email administrators who forward email to Gmail from other servers or services. Follow these best practices to help ensure Gmail's spam filter correctly classifies forwarded messages.
Forwarding and spam
Avoid changing the envelope sender when forwarding email to Gmail.
Some email software or services change the envelope sender to your domain when forwarding messages. If the envelope sender is changed to your domain for forwarded spam, Gmail might learn that your domain sends spam. Gmail might treat future messages from your domain as spam.
You can use third-party software to identify spam message and prevent them from being forwarded. If you never forward spam to Gmail, you can change the envelope sender to your domain.
Procmail typically changes the envelope sender for forwarded messages. To fix this issue, add the following to your Procmail configuration file:
SENDER=`formail -c -x Return-Path`
SENDMAILFLAGS="-oi -f $SENDER"
We recommend email administrators always set up email authentication for their domain. However, authentication can affect message forwarding.
Don't modify message headers or message body for forwarded messages. If the body of the message is changed, messages won't pass DKIM authentication and might be sent to spam.
Other reasons that forwarded messages fail DKIM include: Modifying the mime boundaries, third-party software modifying the body of the message, expanding the message recipient using LDAP, and re-encoding the message.
Gmail has strict authentication for frequently spoofed domains, such as eBay, PayPal, and Google. If your mail server modifies forwarded messages from these domains, Gmail might mark them as phishing. Forwarded messages from these domains have an Authentication-Results header that shows DKIM failed. Messages sent directly to Gmail from these domains pass DKIM.
Forwarding with a mail provider
To prevent spam from being forwarded to Gmail, update your mail provider's forwarding settings. If you can't modify the forwarding settings, contact the mail provider for help.
Your users can update their Gmail Accounts settings to prevent Gmail from displaying a phishing warning for forwarded messages. In the Accounts tab, enter the email address being forwarded to the Send mail as setting. For detailed instructions, go to Send emails from a different address or alias.