Emails bouncing as undeliverable - to gmail accounts

[#1]

I use Plusnet smtp to relay messages from my onetel account. This works for most people (I went through account validation with Plusnet years ago when I joined) but recently I've had send failures like this replying to incoming messages from gmail and nub.news domains. Any ideas as to why and how to resolve the issue?

      This is an automatically generated Delivery Status Notification.      

Delivery to the following recipients failed permanently:

   * [valid recipient]@gmail.com

Reason: A message that you sent to the following recipient could not be delivered due to a permanent error. ** The remote server ?? responded with: ** [valid recipient]@gmail.com ??:?? This message was created automatically by mail delivery software on the server .


Reporting-MTA: dns; avasout03 [84.93.230.244]
Received-From-MTA: dns; [192.168.1.7] [213.31.174.118]
Arrival-Date: Fri, 07 May 2021 11:42:32 +0100


Final-recipient: rfc822; [valid recipient]@gmail.com
Diagnostic-Code: smtp; 550-5.7.26 Unauthenticated email from onetel.com is not accepted due to domain's

Last-attempt-Date: Fri, 07 May 2021 11:42:32 +0100

Also another :-

Final-recipient: rfc822; [received sender name]@nub.news
Diagnostic-Code: smtp; 550 5.7.1 Email rejected per DMARC policy for onetel.com

Last-attempt-Date: Wed, 28 Apr 2021 22:51:11 +0100

[#2]

It sounds like you're using Plusnet's servers to send a message with a From: address of your OneTel account? If so, the anti-spoofing policy of OneTel's domain doesn't include Plusnet as an authorised sender hence Google are rejecting the message. The remedy is to use OneTel's servers (at TalkTalk now?) to send out mail.

[#3]

I've been using plusnet smtp relay that way for seven years. I vaguely remember I couldn't access onetel's smtp server from plusnet. Plusnet had some verification procedure I had to go through to be able to send "as from" a onetel's address Can't remember the detail now, but it worked fine for five years. The problems I have now started 6 months or so ago, hard to say when because not all servers send a bounced error message. Something has changed somewhere and it's a pain. I don't think it can be the SPF record thing from two years ago at Plusnet. Maybe Onetel's did the same more recently?

[#4]

In general, if you have an email address me@somewhere.com you ought to send your outbound email via smtp.somehwere.com NOT via a third party's SMTP service.

If your (or the) DNS settings for settings for somewhere.com do not specify that relay.plus.net is a legitimate sender for @somewhere.com then there is the risk that the RECEIVING email service might reject the email because relay.plus.net is not a valid MX submission service for that domain.

None of this has anything to do with Plusnet.  The domain, its SPF rules and the rigor with which the receiving domain treats email relayed by Plusnet on behalf of an external domain are all third party issues.

The best advice available is use the correct SMTP server for the email address you are sending from.  Do not expect how you bent the light before to always bend the light the same, as good security measures become tighter.  Real credit to the email service which is bouncing these emails and the email host service which is specifying SPF restrictions - they are key measures to mitigating sender address spoofing.