Hello Martin,
It looks like we finally solved the problem. I found this discussion https://moodle.org/mod/forum/discuss.php?d=321521 that made me check the version of phpmailer.
The version of phpmailer that I had was v5.2.7 and in the previous link you can see that this version has a problem with TLS encryption.
We replaced class.smtp.php and class.phpmailer.php with the once included in Moodle 3.1 (phpmailer v5.2.14) and it worked.
One note, this also worked having "always send from no-reply" checked and the support account email has to be the same as the email username you are loging to the smtp.
I hope I had helped.
Regards and thanks to all!
Leandro