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Simon,

Reason: Remote SMTP Server Returned: 500 Invalid request

I'd send that one to [email protected] - you may need to send a more recent instance (if you have one) so that Mark can find the event in the server logs.

When I resend a few minutes later it works, this has happened a few
times now, and only when sending to groups.io, so I’m wondering if the
groups.io mail server can get overloaded?

Possibly that, or possibly some sporadic miscue between your email server and Groups.io's servers based on timing or other factors.

If it were purely an overload situation then I'd expect that other people would experience it as often as you do; I never have and there haven't been a lot of reports like yours.

Shal