Silly me. Hetzner not in the sending loop. Box is hosted with you, but my emails are with Hetzner.
The program creates an email, To: user1, CC:user2 and myself.
User1 and User2 are at same domain, not mine.
It *looks* like the mailer is sending the messages to user1 and 2 first, via hornet, and then to me, via my server. So I assume hornet is in their mail loop, which is odd because they are MS Outlook users. Thought mail was with MS. Mails from them come from outlook.com but I see mx-relay05-hz1.antispameurope.com in the headers which has a similar name pattern to mx-gate80-hz1 below.
This is from mail log (edited):
Aug 1 10:18:58 (servername) sendmail[31672]: STARTTLS=client, relay=mx01.hornetsecurity.com., version=TLSv1/SSLv3, verify=FAIL, cipher=ECDHE-RSA-AES256-GCM-SHA384, bits=256/256
Aug 1 10:19:00 (servername) sendmail[31672]: 2718IufQ031670: to=<
[email protected]>,<
[email protected]>, delay=00:00:04, xdelay=00:00:04, mailer=esmtp, pri=183441, relay=mx01.hornetsecurity.com. [94.100.132.8], dsn=2.0.0, stat=Sent (OK accept as A7CAC1200316:d1a268ef6e701a6a46d0264affec88d9 by mx-gate80-hz1)
Aug 1 10:19:01 (servername) sendmail[31672]: STARTTLS=client, relay=(my domain), version=TLSv1/SSLv3, verify=FAIL, cipher=DHE-RSA-AES256-GCM-SHA384, bits=256/256
Aug 1 10:19:01 (servername) sendmail[31672]: 2718IufQ031670: to=<
[email protected]>, delay=00:00:05, xdelay=00:00:01, mailer=esmtp, pri=183441, relay=(my domain). [xxx.xxx.xxx.xx], dsn=2.0.0, stat=Sent (Ok: queued as 4B3DD1D06727)
So "probably all okay", just not what I was expecting.
Thanks, Ian