Getting error while sending DB mail from SQL SERVER 2008 R2(RTM)

Hi All,

We are getting error while sending DB mail at sql server 2008 r2(RTM) and SQL Server 2008 R2(SP2).

We are working in Cluster environment, previously it was working fine, suddenly from last night it is not working.There is no change in set up. We are getting below error:

The mail could not be sent to the recipients because of the mail server failure. 
(Sending Mail using Account 5 (2016-02-15T00:58:47). 
Exception Message: Cannot send mails to mail server. (Failure sending mail.). )

But we are able to send mail with same SMTP ip at same server by telnet. 

Please help here and let me know if  any other information required.

Thanks