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I heard somewhere that I can mark a mailto: link somehow (maybe it was the email address itself that contained some flags) so that when I receive an email, it will already have some hidden marker, which can be useful when leaving one email address on many different websites (you can see from where the email came).

I think it had something to do with embedding the topic of the email into the email address or using dots in it.

I'm trying to find a way to make it easy to distinguish emails sent from different places.

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up vote 2 down vote accepted

What you're looking for is the +. So, for example you could sign up with an e-mail address like:

Here, your mail will be delivered to, but the "To" field will show the identifier you added after your name as well—in that case, foo. This is called "sub-addressing".

This trick has been known for quite some time now, an early reference being this LifeHacker article from 2005 where it became popular for the Gmail service which was quite new (and invite-only) at that time.

You can read more about allowed characters in e-mail addresses in RFC 3969 and the actual sub-addressing technique, which is explained in RFC 5233.

Note that this does not work for all mail services. The service has to support this kind of addressing.

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