You would like to know if Symantec Mail Security for Exchange (SMSMSE) has any involvement with mail flow or mail processing when the SMSMSE service is stopped.
When the SMSMSE service is stopped, the product will have no involvement in the processing of an Email by Microsoft Exchange. It cannot scan an Email that is received by Exchange at this time.
If an Email was received by Exchange when the SMSMSE was stopped on a server with a mounted Exchange Messaging/Information store, upon SMSMSE being started again, the product can scan the Email for viral threats upon the next instance of the Email being accessed by a user in their Mailbox (where the Email resides on the Messaging store), it can also scan the Email with a Scheduled,Background or Manual scan. The product cannot scan the message for Spam violations at this time, as the detection of these threats is only possible during the initial delivery of the Email to the Exchange server via the SMTP protocol.