first of all thanks for your suggestions.
recently i saw outgoing mail log file their i can find : javax.mail.AuthenticationFailedException: 454 4.7.0 Too many login attempts, please try again later. this error.
When i edit the test mail and test the connection that time it gives me above error and if i log out and login JIRA then i tried to test connection then it shows connection successful msg.
Steven's suggestion of a restart is to help you debug it, and get the queue restarted for this one incident. It's not a permanent fix.
You really do need to read the application log to find out what is going on, as well as the logs generated by the debug flags you've set in your first comment.
Have a look at the first "stuck" email in the queue and examine the log for that point. And after the restart look through the log from start to where it's active and working looking for any warnings and errors on related to email.
We can't give you any solution until we've worked out what the actual problem is, and the logs are your best way to find that out.