Mailgun api key, what is it for?

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Configure VERP to handle bouncing e-mails

Setting up bounces handling Discourse uses the Variable envelope return path (VERP) technique to handle bouncing e-mails. To enable VERP, set the “reply by email address” site setting with an email address template that uses the + sign. The default of replies+%{reply_key}@discourse.example.com … works fine. If you are using your own or your company’s email server, then, that’s it Your discourse instance must be able to receive email sent to this address. This should work via POP, bu…