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I'm wary (used it right this time Shal!) of changing this, ...Indeed. ;-)
I too do not favor changing it.
Or even aliasing it. I thought of that but realized it wouldn't solve the problem - the people who need to use the alias wouldn't know about it. That way lies only more confusion.
If I had to do it over, I'd probably special case this to handle it,I'm not sure what you're thinking, but I thought perhaps you could notice when receiving a message:
IF (message delivered to Groups.io by btinternet.com) AND
(Received chain includes synchronoss.net)
Do something different.
The question is what to do. I initially thought you could look to the header To and/or Cc fields to determine if there was a command and what it was, but I think some of the messages I looked at had neither. Probably cases of Bcc, but I'm not sure about that.
So the only two actions I can think of would be to either force the message to Pending, and let the mods deal with it, or reject the message, with a message text saying that btinternet users must post via the web interface, not via SMTP client applications.
But either of those are pretty onerous for the 99.9% use case of a simple message posting. So maybe accept the message for posting if there is a To or CC that includes the group posting address without command.