not working under 2k19 std server

[rnervi]

(under a win10 it works sampe scripts)
credentials has been checked twice.
using a gmx account, so well known.
here the debug log:

2024/07/21 11:54:06 - mailsend-go v1.0.10 starts -
2024/07/21 11:54:06 Subject: test
2024/07/21 11:54:06 From: ***@gmx.com
2024/07/21 11:54:06 To: ****@gmail.com
2024/07/21 11:54:06 Subject: test
2024/07/21 11:54:06 From: ****@gmx.com
2024/07/21 11:54:06 To: *****@gmail.com
2024/07/21 11:54:06 SMTP server: smtp.gmx.com
2024/07/21 11:54:06 SMTP Port: 465
2024/07/21 11:54:06 Setting From with name: notiche.omec@gmx.com,
2024/07/21 11:54:06 SMTP server: smtp.gmx.com
2024/07/21 11:54:06 SMTP Port: 465
2024/07/21 11:54:06 Setting From with name: *****@gmx.com,
2024/07/21 11:54:06 Using ESMTP Authentication
2024/07/21 11:54:06 SSL? true
2024/07/21 11:54:07 ERROR: 535 Authentication credentials invalid

some suggestions ?