This is something you need to check on the mail server side. This message tells you that the mail server you are connecting with for some reason broke off the connection.
Regards,
Ronald
Hello Girish,
what is your use-case in terms of using authentication? Are you using basic credentials or using OAuth. It seems the error is coming from your mail provider / mailserver, so investigate from that point of view, if there are no useful message in Mx console.