Yes, this question is a duplicate to: SSRS - rsAccessDenied error but that question is 2 years old and unanswered...so I thought I might try again.
Our DBA has recently departed leaving behind a number of SSRS report subscriptions to which he is (or was) the 'owner'. Based on the advise found at Changing the SSRS Subscription Owner I have created a new domain account, assigned that domain account full privileges on the SSRS server (at least 'for now') and yet I am still getting the error message:
Failure sending mail: The permissions granted to user 'DOM\NewUser' are insufficient for performing this operation.Mail will not be resent.
What privileges, in SSRS, SQL Server and/or Active Directory are required to allow "NewUser" to send email.