emails rejected by gmail

[#1]

I know that this topic has been raised before but I am still unclear how to resolve the issue. I am responding to an email using reply all - one of the recipients has a gmail.com account.

I receive the response below indicating that my email is rejected. It rather suggests that it is my plusnet address [myplusnetname.plus.com] which needs amending but I have no idea how to do this. Can anyone point me in the right direction?

This is the mail system at host relay.mailchannels.net.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
                   The mail system
<redacted to address>: host gmail-smtp-in.l.google.com[142.250.99.26] said:
    550-5.7.26 Your email has been blocked because the sender is
    unauthenticated. 550-5.7.26 Gmail requires all senders to authenticate with
    either SPF or DKIM. 550-5.7.26  550-5.7.26  Authentication results:
    550-5.7.26  DKIM = did not pass 550-5.7.26  SPF [my plusnet name.plus.com] with
    ip: [23.83.220.6] = did not pass 550-5.7.26  550-5.7.26  For instructions
    on setting up authentication, go to 550 5.7.26
    https://support.google.com/mail/answer/81126#authentication
    d9443c01a7336-21f368bcc9csi123489935ad.432 - gsmtp (in reply to end of DATA
    command)

Reporting-MTA: dns; relay.mailchannels.net
X-Postfix-Queue-ID: 83DF6322BBB
X-Postfix-Sender: rfc822; address@my user name.plus.com
Arrival-Date: Mon, 10 Feb 2025 07:53:45 +0000 (UTC)

[#2]

550-5.7.26  DKIM = did not pass
550-5.7.26  SPF [my plusnet name.plus.com] with ip: [23.83.220.6]

Are you sending this email via relay.plus.net?

Are you sending the email DIRECT to the Gmail addressee?

The mentioned IP address [ 23.83.220.6 ] is someone else's relay service [ aye.yew.relay.mailchannels.net ] which is clearly not a permitted sender for @accountname.plus.com

Plusnet implements DKIM for all @accountname.plus.com addresses - the above reports that DKIM failed and SPF was checked ... but with a non Plusnet relay server address.

A logical conclusion might be

1. You are not using Plusnet's relay server (relay.plus.net) to send this email
2. The addressee you are sending this email to is not @gmail.com, but rather some other address and ...
   1. They are forwarding mail sent to that address to their Gmail account
   2. Their email hosting / forwarding provider is not implementing SRS

SRS is a mechanism which enables the final receiving service to understand that the the ORIGIONAL sender did NOT use the middle man's relay service to send the email.

Without SRS it looks as though YOU sent the mail using the addressee's redirection service.

If you can clarify if either of these scenarios are relevant, further advice can be provided.