emails to gmail addresses undeliverable due to DMARC Authentification issues

[Maartinf]

Hi, first post here.

First a little rant, I've had to register under my partners surname as my own is rejected as containing unacceptable language (F-A-Gan)! unbelievable!

But to my main gripe, I was sending an email to the members of our small charity committee this morning (8 people, all on "domestic" email addresses) using Thunderbird and my own domestic btinternet address.  The two gmail addresses both gave this response;

SMTP error from remote mail server after end of data:
    550-5.7.26 Unauthenticated email from btinternet.com is not accepted due to
    550-5.7.26 domain's DMARC policy. Please contact the administrator of
    550-5.7.26 btinternet.com domain if this was a legitimate mail.

Authentication-Results: fwd1.hosts.co.uk;
	iprev=pass (mailomta22-sa.btinternet.com) smtp.remote-ip=213.120.69.28;
	spf=pass smtp.mailfrom=btinternet.com;
	dmarc=pass header.from=btinternet.com;
	arc=none
Received: from mailomta22-sa.btinternet.com ([213.120.69.28] helo=sa-prd-fep-042.btinternet.com)
	by fwd1.hosts.co.uk with esmtp (Exim 4.97.1)
	(envelope-from xxxxxx.xxxxxxxxxxxx@btinternet.com>)
	id 1rPMs9-0000000084f-0r4L;
	Mon, 15 Jan 2024 13:19:25 +0000
Received: from sa-prd-rgout-005.btmx-prd.synchronoss.net ([10.2.38.8])
          by sa-prd-fep-042.btinternet.com with ESMTP
          id <20240115131907.WPPH20040.sa-prd-fep-042.btinternet.com@sa-prd-rgout-005.btmx-prd.synchronoss.net>;
          Mon, 15 Jan 2024 13:19:07 +0000

[Maartinf]

I should have added

What the heck does this all mean, and how do I resolve it?

[licquorice]

Are you using your own domain's server to send btinternet email?

BT's DMARC authentication is fine.

[Maartinf]

No, I'm a total numpty with this stuff so all settings (in Thunderbird and Btinternet) are default, so I assume the server is standard BT

[licquorice]

Is it just those 2 Gmail addresses or do all Gmail addresses give an error?

[Maartinf]

Hard to say, as these are the only 2 of my regular contacts who use gmail.  Last month (last committee meeting) their addresses worked just fine, as do others on a music group I'm with

[licquorice]

Not sure where the fwd1.hosts.co.uk is coming from, there's nothing from Gmail. Are the recipients forwarding their Gmail to own domain addresses?

[Maartinf]

yes, they both have a private domain at me.uk, is this the issue? If so what can we do? 

[licquorice]

They need to get their domain provider to configure their DMARC, SPF and DKIM settings correctly, but it all seems to be back to front as it is you sending not them.

[Maartinf]

I'll get onto Bob to check his end, once he finds a grandson to explain the request!  I still don't understand why its happening unless he's clicked some "maximum security" setting.