Error in E-mail Notification in BPM

Hi folks,
  I have create one Simple BPM Project in which i Created one Notification Activity to send the mail
  but when i am starting my BPM Project it is showing the below error in history :

Notification could not be sent to
the following recipients:
xxx@xxxx.com(Error sending e-mail),
xxx@xxxcom(Error sending e-mail)

and also the Task Event Notification is not working , giving following error :

Event notification could not be
sent to recipients xxx@xxx.com for task instance ID
7800745f-3a34-11e4-b4c2-0000037bb076. Problem: Cannot send e-mail because the
e-mail session cannot be retrieved

Please give the solution for this.

Thanks