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Google bounces all URLs through a redirector which strips referrer information and also allows them to warn about malware sites that were identified after the message itself was classified and delivered.

I would like to test that but when clicking different links I get mixed results. The URLs get modified alright but a lot of referrer info seems to not get removed.

I guess its just Google collecting one more metric.

It's for security. Checking against a list of banned URLs, etc. When you have user-submitted content, protecting your users from bad URLs is important.

Also, I'm sure they're doing plenty of click tracking too :)

i think this is to remove real referer url from header, as this behavior is also present when you use web version of Gmail.

but with that solution tracking is also possible