Smtp configuration with mailjet in an impasse


I use owncloud (8.2.7 until a save upgrade plan to 9.x) installed on a NAS.
I try to configure the mail server to use the smtp service of mailjet. And i fail.

When i use TLS / 25, i get the error when i try the test email:
Erreur: Failed to authenticate on SMTP server with username " 3d494c... " using 1 possible authenticators

As you imagine, i have double checked the parameter and if the port is open.
Could it be a problem with owncloud? The server?
How can i check the log?

Thank you in advance for any help.


troubleshooting steps are available here:

@RealRancor Thank you for the link.
I will check it.