Thank you for the help.
I found out the issue.
Explanation:
I was creating an email delivery using some fragments and i was getting error in the preview,creating thumbnail and publishing.
When any email delivery is created using fragments/templates the ID of the fragment/templates is used in the html of the email.
Fragments in my dev instance were renamed and when i was trying to preview email, template had old naming convention and at time of preview/publish new names were referred.
So there was a conflict in the naming convention.
Thank you