SMTP authentication strange error

[KanielOutis]

My E-Mail Notifications fail with an:
"Login denied: gnutls_handshake() failed: An unexpected TLS packet was received."
Sometimes it is an :
"Login denied: _client_version10.1.26 _platformx86_64" Error.

My SMTP Configuration:

SMTP server = mail.domain.com
SMTP server port = 587
SMTP helo = mail.domain.com
SMTP email = [email protected]
Connection security = STARTTLS
SSL verify peer = unchecked
SSL verify host = checked
authentication = normal password

What should I do to solve this Problem?
Thanks your for help

[KanielOutis]

I think there is a problem with gnutls.
If I run "gnutls-cli -d 5 mail.domain -p 587" I receive an error.
If I run "openssl s_client -connect mail.domain:587" it's succesfull.

Is there a way to disable gnutls at zabbix and enable openssl without reinstalling zabbix?