Why is SPF being validated against my mail server's IP instead of sender's IP?

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http://serverfault.com – I have a mail server "example.com" which forwards all emails with recipient "[email protected]" to "[email protected]". My mail server runs Postfix and it uses the virtual_alias_maps mechanism to perform the forwarding. I also have SPF records installed for "example.com": v=spf1 a include:aspmx.googlemail.com ~all The problem is, whenever someone delivers mail to "[email protected]", Gmail validates the example.com SPF records against example.com's IP address! I thought it's supposed to validate against the original sender's IP address. For example, I'm on my laptop on my home Internet connection. I (HowTos)