Owner approval request using Distribution List without Exchange Server

1. Ower approval request in DL management is not completed it is in authorizing status while check in Search Request, and user will not be a member in requested DL when request sent by individual owner however it is working while i am using service account, we are not using Exchange with FIM for notification. i have also checked Event viewer and found some errors related to Email notification

i.e. warning :

WorkflowInstance 'cb25e1ff-4559-4b3f-87bb-47d24e60a87e' could not send mail message in activity 'approvalActivity1.sendApprovalEmail'.

Error:

If anyone have any idea it will be really helpful.

Thanks - Ankit Gupta