Summary
Replies back to a Mattermost instance’s email messages fail to get through, then external email error messages occur.
Steps to reproduce
I replied to a self-hosted in-cloud mattermost’s three test messages though both yahoo and gmail accounts, with the same results.
Mattermost Enterprise Edition
Mattermost Version: 9.5.3
Database Schema Version: 119
Build Number: 8427860509
Database: postgres
Expected behavior
I expected that replies to the sending address ‘admin@<mm.example.com>’, would come into mattermost.
Observed behavior
Gmail error message:
550 5.1.1 : Recipient address rejected: undeliverable address: host fr-int-smtpin5.hostinger.io[private/dovecot-lmtp] said: 550 5.1.1 User doesn’t exist: admin@‘mm.example.com’ (in reply to RCPT TO command)
Final-Recipient: rfc822; admin@‘mm.example.com’
Action: failed
Status: 5.1.1
Remote-MTA: dns; mx1.hostinger.com. (2606:4700:90:0:c1f8:f874:2386:b61f, the
server for the domain ‘mm.example.com’.)
Diagnostic-Code: smtp; 550 5.1.1 <admin@‘mm.example.com’>: Recipient address rejected: undeliverable address: host fr-int-smtpin5.hostinger.io[private/dovecot-lmtp] said: 550 5.1.1 <admin@‘mm.example.com’> User doesn’t exist: admin@‘mm.example.com’ (in reply to RCPT TO command)
Last-Attempt-Date: Mon, 06 Jan 2025 11:55:28 -0800 (PST)
Yahoo error message:
Sorry, we were unable to deliver your message to the following address.
<admin@‘mm.example.com’>:
550: 5.1.1 <admin@‘mm.example.com’>: Recipient address rejected: undeliverable address: host fr-int-smtpin1.hostinger.io[private/dovecot-lmtp] said: 550 5.1.1 <admin@‘mm.example.com’> User doesn’t exist: admin@‘mm.example.com’ (in reply to RCPT TO command)
---------- Forwarded message ----------
OK - - - Test test.
Brian
On Monday, January 6, 2025 at 02:56:40 PM EST, admin@<mm.example.com> wrote:
Brian
This appears now to be due to badly-set-up Mailermost IMAP settings.
Is the Reply-To: literally admin@‘mm.example.com’?
And I’d recommend upgrading to 9.11.6 - I very much doubt that will solve this issue, but you’re missing a lot of bug and security fixes, and 9.5.x is already EOL.
John, it is not - I thought that hiding the actual address was expected for security.
Thanks for the upgrading advice. On advice, do you have any general advice on how to set up mattermost with mailermost for an organization’s email needs that about once a month exceeds 500 emails an hour, the current listserv provider limit; without spending inordinate amounts of time on set-up and admin, and staying up until 3am like I did last night trying vainly to get gmail delivery working? (Would you believe I’m asking for a friend?) It looks like one can no longer host a server from home at my skill level, and self-hosting in the cloud has been a huge time-sink and failure.
So, first, I’m told that Mailermost does not support email replies yet… which implies it may be coming.
And I didn’t mean that mm.example.com was literally your domain, but the single quotes in there would be an issue, I’d assume!
You should be able to configure Mattermost to use your ISP or cloud providers SMTP server. That’d be ideal. Setting up Postfix isn’t that hard, but there are a lot of things like SPF records, DKIM, DMARC, etc. that, if not configured, make it hard for you to deliver mail, especially to huge providers like Gmail.
I’d also suggest that if you’re doing this for an organization and Mattermost and notifications are critical, that having a paid subscription and support can become a very big deal. Or a hosted Mattermost instance where someone else worries about all of this stuff 
John, thank you for these insights.
As those in our organization have varying degrees of website comfort and ability, integration of emailed replies right into the mattermost forum through mailermost strongly drew interest - some deemed it essential. I’m very disappointed to hear that mailermost is so incomplete.
Are there efforts to crowdfund finishing mailermost and such work?
I put the single quotes into the message, seeking to convey clarity, which escaped me.
SPF, DKIM and DMARC have absorbed much of my time on this, but gmail still isn’t happy, though some messages get through, albeit with errors thrown.
Do you know of time and money evaluations looking at these different ways to run mattermost.
To get more info about a license key or a hosted instance, you can fill out Talk to an Expert | Mattermost I’m guessing you are not a US federal entity, and so outside of my wheelhouse I can help find the right person, but that form is probably the fastest.
Mailermost is not a supported part of Mattermost, it’s a project started by some people. One still works here, and he’s the one who gave me the unfortunate news. It doesn’t look like there’s been a lot of interest in it, but I could ask about the LoE to get it to a state that would be useful for you. Keeping in mind it still wouldn’t be officially supported. GitHub - mattermost-community/mattermost-plugin-email-reply
I know Gmail is very, VERY picky. I volunteer for a 501(c)(3) and have an email address there. I forwarded it to a Gmail account, but it keeps getting kicked out, so I had to forward to another mail service.
You could also add a feature request at https://mattermost.uservoice.com/forums/306457-general I have no idea how they’re evaluated or how many other people might like and vote for it, but I do know it’s watched and there have been features built from it!
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