Steps to reproduce:

1. Login to the Management Panel
2. Administration > Media Types - Change Email Settings
3. Fill the form. Connection Security is None. Authentication is Username/Password. Port 25.
4. Save and Test.
5. Error: Failure when receiving data from the peer.

Result:

Screenshot of the error.

From zabbix_server.log:

 
10630:20201021:131644.320 failed to send email: Failure when receiving data from the peer
10631:20201021:133312.800 failed to send email: Failure when receiving data from the peer

Network traffic:

I can send an email using Telnet from Zabbix Server.

Expected:

I can send a test email. Frontend should use AUTH LOGIN instead of AUTH GSSAPI.