SAP BPM Email Notification is NOT working Everytime

Hello Everyone,

SAP BPM is not sending email notification to user sometime(1 out of 10). Configuration is fine. But, i am unable to understand why this problem occurring. could you please suggest to solve this.

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