What is the purpose these long string reply-to email addresses?

For example Groupon and Facebook seem to use these.

Groupon <reply-fec4e341ec32311-125727_HTML-677829183-96988-0@e.groupon.com>

Reply to Comment <g+41wq2z1j00000000u2kx002eqs1u620s0037hlmwm4621up33@groups.facebook.com>

In the facebook example - it makes sense that I 开发者_如何学编程could be replying to a particular comment thread and this is a unique hash that lets me do so from within an e-mail.

As for the groupon example, I am not aware of the purpose.

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If I had to guess, I'd say Groupon does it so they can tie replies/complaints to a specific campaign. They must send out many millions of mails a day, and probably have many hundreds of campaigns running at the same time. Being able to automatically link feedback to a campaign is probably a huge time saver, and provides useful KPI stats.