Although I haven't seen this message before, I do know that Gmail is tightening up security, as others have posted about difficulty in sending to Gmail. In their cases, the issue was insufficient security on email host servers. That doesn't solve your concern, but I believe your error message received may be part of their heightened security.

I suspect the error message continues with something like: Diagnostic-Code: smtp; 550-5.7.26 This mail is unauthenticated, which poses a security risk to the

That's what I was getting, anyway. For me, a partial fix was to go into Tools, Account settings, then for my default identity enter my email address as given per my network provider, relegating my usual email address to the reply-to-address. Previously I had used [edited] @psaa.me.uk for the Email Address field and left the Reply-to field blank.

Now the recipient gets to see my email as provided by my network provider, but paul@.... comes up when he drafts his response. But at least my email gets through!