Cannot send mails to mail server. (The operation has timed out.)

I have configured database mail in sql server 2008 r2 on my local machine.

While testing I have observed that some of mails are not getting triggered.

Log file shows following message-

The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 7 (2013-04-30T10:23:57). Exception Message: Cannot send mails to mail server. (The operation has timed out.)

I have wriitten on SP for snding mails which sends 94 mail to different ailaddress  out of which only 35 are getting delivered.

Plz Assist me.