Hello Fabio,
thank you for your response.
My host gives the opportunity to use a Let's Encrypt certificate, it's a free service so that's great.
I can choose which domain i want to added to the certificate, like mywebsite.com, mail.mywebsite.com, smtp.mywebsite.com
So i tried with and without smtp.mywebsite.com
I tried every mail option in Joomla, sendmail, php mail and smtp. Without error reporting the email test works for all 3. With error reporting i get the same error messages as you and with all 3 email send types, also the escapeshellcmd php extension is mentioned in the error. My host refuses to turn it on because of a bug you can misuse.
Now on the joomla forum it is confirmed that version 3.7.0 will have the latest PHP mailer 5.2.20, where the vulnerable bug is fixed.
See
forum.joomla.org/viewtopic.php?f=706&t=947605
So most probably the issue is with the certificate. And the bug is only when using mass mailer from CB, but also Joomla's own.
Not the single registrations emails.
The host is offering a workaround, but they need my admin login for Joomla. I'm not happy with that, there is private info on my site from users.
So i'd like them to tell me what the work around is. I can do it myself.
If it's just setting SMTP, i already tested that.
Thanks for you help so far, i appreciated it.
Hopefully it will be solved soon.
Kind regards,
Jerry