Question Mail delivery failed: returning message to sender

Thanks - on the page about K6 it says it is for Joomla 4 is it ok to use on J3 though?

It will not work with J3.x

OK thanks in that case i am still looking for a Kunena fix for the " Mail delivery failed: returning message to sender " that i get on almost every subscriber notification from my forum

all of them say:

host gmail-smtp-in.l.google.com [108.177.xx.xx]
SMTP error from remote mail server after end of data:
550-5.7.1 [78.129.xxx.xx] This message is not RFC 5322 compliant, the issue is:
550-5.7.1 duplicate To headers. To reduce the amount of spam sent to Gmail,
550-5.7.1 this message has been blocked. Please review
550 5.7.1 RFC 5322 specifications for more information. b1-20020adff241000000b0023501eafabcsi3615830wrp.69 - gsmtp

Have you run one of the emails through one of the free DMARC checkers to see what they come up with. (Try mxtoolbox.com to start though there are better ones.) Unless compliant your mail will be rejected. I don't know if you use a hosting company but it could be as simple as the text file not being set up properly so the check doesn't work.

Thanks for your reply (i didnt get a notification ironically!)
So it turns out this is a bit of a minefield and the instruction appears to require considerable knowledge which i dont have!

prnt.sc/J3PjbUuQsg3r

how do you set a dmarc policy? this seems like mxtoolbox trying to sell their software?

sites like www.mail-tester.com/ show 10/10 but these are emails coming from Outlook not Kunena. How do i replicate or even view the emails that come from the forum which are bouncing constantly?

thanks

First of all DMARC has nothing to do with MX-Toolbox but they will sell you their services to set this up, though I have always done this myself.  DMARC is an industry standard to try to stop spoofing and to check that they are coming from the correct person,organisation, etc,  (One of the issues with Joomla was that it sent out emails from one account though the email showed as coming from another, and that can cause issues.  That may have been resolved. ) DMARC.org is behind the system, and included many big names in the Internet, IT, and financial industries.  Outlook and Google are strong adherents to it, as are Paypal, and will reject non compliant emails.  Most others, including hosting companies are the same.  Usually your hosting company will set this up on your behalf these days along with the email accounts, so you will need to check.  If this is the case then that will have nothing to do with either Joomla or Kunena.  I note that the output from your clip does not mention the DKIM or SPF record that are required so I assume that they have not been set up by your hoster
Have a look at this link to see if it helps with advice:
powerdmarc.com/dmarc-policy-not-enabled/
cyberpanel.net/blog/dmarc-policy-not-enabled-cloudflare/

Thanks for the tips.
The tester says that my DMARC policy "has been correctly set up"
SPF is correct and DKIM also - both are shown as "valid" in Cpanel and they show as 10/10 on mail-tester.com

I have changed the DMARC policy to v=DMARC1; p=quarantine; just to see if that makes any difference.

I have also been playing around with the settings in the USERS tab of the config in Kunena. There is a field "E-mail to Multiple Recipients" which was initially set to 50 which i assume meant that emails were sent out with bcc up to 50 emails. I changed this to 5 which didnt make any difference and now to 0 - but im not clear if that means 1 email at a time is sent or all of them are bcc'd !
The message from Google is "550-5.7.1 duplicate To headers" - if it was a dmarc issue wouldnt it say that?

thanks for your help with this.

PS - the notifications from this forum are marked as spam!!
From: "Forum" <[email protected]>
To: <[email protected]>
Subject: ***SPAM***  Mail delivery failed: returning message to sender (General questions and how-tos)
Date: Thu, 22 Dec 2022 17:46:46 -0000
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: multipart/alternative;
    boundary="----=_NextPart_000_0166_01D916A6.7D6E67B0"
X-Mailer: Microsoft Outlook 16.0
X-Spam-Status: Yes, score=5.1
X-Spam-Score: 51
X-Spam-Bar: +++++
X-Spam-Flag: YES

So frustrating, am i the only one with this problem? I have even tried looking for alternatives to Kunena - any suggestions are gratefully received!

Hello,

I have found that : www.reddit.com/r/gsuite/comments/yjlyv7/...rom_certain_address/

kunena.org will move soon to K6.0 and Joomla! 4.x so we will see if there are changes on this side

Yes thanks it all used to work fine but for the last 4/5 months or so i get all gmail emails sent through Kunena returned, it obviously isnt just my forum.