SMTP Error during Notification email sending

Hello,

There is currently a stange issue with sending email notifications to the SpiraTest users. On notification event the following error is detected in System Logs:

Syntax error, command unrecognized. The server response was: Callback from SMTP Send:
In APPLICATION.Common.MailClient::smtpClient_SendCompleted():

Messages:
Syntax error, command unrecognized. The server response was: [System.Net.Mail.SmtpException]

Stack Trace:
at System.Net.Mail.SmtpConnection.ConnectAndHandshakeAsyncResult.End(IAsyncResult result)
at System.Net.Mail.SmtpTransport.EndGetConnection(IAsyncResult result)
at System.Net.Mail.SmtpClient.ConnectCallback(IAsyncResult result)
Callback from SMTP Send:

 After contacting the SMTP Mailing provider, their response was that they are only seeing the Syntax error on their side.

The Email configuration involves SSL and the mailing server can be connected from the hosting machine and the certificate is considered as a valid one.

SpiraTest version is 6.0.0.3 with all the modules of version 6.0.0.0.

I'd like to know if anybody has faced the same issue and what could be the issues or maybe it is a system bug?