Reporting Server Subscription Issue

Dear All,
We are facing some issue in reporting service subscriptions. In one of the
server when We create subscription to save or send report, Its giving below
error.
But reports works fine directly from the report manager.
ERROR:
Failure sending mail: An error has occurred during report processing.
Its not logging anything on the report server log file. We have configured
reports server's .config files to point to application's dll file.
Everything works fine from the report manager.
We created a report with some data from the database which is working fine
with the subscription. But when I added code to access application dll, its
not working through subscription
(though it works from report manager).
Anyone faced similar issue and able to solve, please let me know.
Thanks,
RamWhy would you change the config in such a way?