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Jim,

Some of my confusion comes from your reference to "talktalk servers".

They came up because the IP address that delivered your message to Groups.io belonged to them.

My ISP, through which I send emails, is Talktalk Residential (who use
the talktalk.net domain).

That's the one I was referring to.

In addition, I had not heard of either SPF or DKIM before and I'm not
sure of the significance of either of them.

They are email authentication standards. Plus a newer one, DMARC, which is built on those two. Most receiving services take one or more of these tests into account when deciding whether to deliver a message to your inbox or to your spam folder, or not at all (reject, aka "bounce" it).

... but since everything is working fine (and has been for 22 years
now) I'm not proposing to make any immediate changes.

Most receiving services will make allowances for the fact that not every legit sender uses these newer (newer than email itself anyway) standards that have been put in place to stem the tide of viruses, robo-spam and other abuses. Groups.io, for example, doesn't currently use those tests on inbound messages - which left the door open to the abuse the OP (Gilly) suffered.

So Mark is currently trying to work out how to tighten up the inbound security, without impacting legitimate traditional use cases such as yours. My fingers are crossed that he'll be able to do something that is effective but low- or no-impact for use cases like yours.

Shal


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