(This might be a feature request rather than a bug, depending on your view…) Consider the following chain of events: A user enabled with Mailing List mode enabled, but excluding own posts User sends in a new topic by mail (say, to support@discourse.my.org), obviously a new topic is created No email is sent back to the user due to their settings User now sends a second mail in The only mail in their email client is the original one in Sent, as no other mail has arrived This also goes to support@discourse.my.org Creates another new thread instead of appending to the first one Given that the second mail will have an In-Reply-To header of the first mail, is it possible to detect that these should be on the first thread?

By default Discourse needs to reply to a special email address that contains a reply key. The reply key is generated when Discourse sends you a notification about a new post and it is only valid for you 1) and that specific post. That’s how Discourse verifies that you are who you claim to be (because nobody else should have access to that reply key) and it also knows to which post you are replying. Such an email address looks like this: support+148425ef5b96ce547e9d79fb3b605760@example.com Without a reply key, Discourse assumes that you want to create a new topic. But, there is a site setting which allows you to change that behaviour. Discourse will try to find related posts by Message-ID when you disable find_related_post_with_key in the settings. :warning: There is a reason why that setting is enabled by default. Disabling it makes incoming emails a little bit less secure. I’d say this is neither a bug nor a feature request :wink: 1) There is one exception to that rule. When you reply to Discourse using a reply key and include other users via To or CC, Discourse knows about that and allows incoming emails using the same reply key from those users.

Aha, so there is a setting, interesting. I see it says “WARNING: disabling this allows user impersonation based on email address” - how does this happen? Presumably it’s some kind of header shenanigans?

Email spoofing Email spoofing is the creation of email messages with a forged sender address. The term applies to email purporting to be from an address which is not actually the sender's; mail sent in reply to that address may bounce or be delivered to an unrelated party whose identity has been faked. Disposable email address or "masked" email is a different topic, providing a masked email address that is not the user's normal address, which is not disclosed (for example, so that it cannot be harvested), bu Th...

Right, I’m aware of spoofing the From header, but I feel I’m still missing a piece of the puzzle so apologies if I’m dragging this out. How does the use of a dedicated To address (reply+token@myinstance) help detect / block From header forgery? Or to put it another way, how does checking the message-id enable that vector?

It doesn’t prevent spoofing of the From header. But the reply key is like a secret that is only known by Discourse and you. Every email sent by Discourse uses a different key. So, unless someone gains access to your emails or you forward the key to someone, nobody should be able to send an email to Discourse and successfully pretend to be you.

Ah, the key is unique per recipient rather than per post? That’s the piece I was missing. Thanks!

It’s actually unique per email sent (so unique per recipient and post).

Reply own mail creates new topic

Product Support

gwmngilfen (Greg) November 22, 2017, 10:03am 9

Ugh, yes, I got that but my comment is misleading :). Thanks for clarifying!

Topic		Replies	Views
Incoming email replies do not share the same topic Product Support email-in	6	406	December 9, 2025
Email-in replies thread wrongly Bug	17	6923	June 22, 2017
Reply by email address broken Bug	7	1222	November 18, 2022
We’re sorry, but your email message to didn’t work Product Support	3	674	March 9, 2019
Send by email a message to add it in a topic Feature email-in	0	545	October 3, 2022

Reply own mail creates new topic

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