Problem with database mail

After long time of successful operation, database mail stop working. The text of the error that is seen in database mail log
The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 6 (2015-12-03T12:24:29). Exception Message: Cannot send mails to mail server. (The remote certificate is invalid according to the validation procedure.).
Mail server (outgoing )
Port 587, SSL=YES. Nothing is changed on account configuration.
Sql server version 2005 Microsoft SQL Server 2005 - 9.00.5057.00 (Intel X86)
What seems to be the trouble?
Mar 25 2011 13:50:04
Copyright (c) 1988-2005 Microsoft Corporation
Standard Edition on Windows NT 5.2 (Build 3790: Service Pack 2)


see my response in this thread
I am sending bulk email from sql server db mail. but suddenly mail not send and receive the remote server is invalid according to validation process. why? using Pls help me