How to identify a process that is spamming?

Question

Fight against spam

Hello!
Spam is sent from different computers on the network to external addresses. We learn about this after the Mdaemon server places these messages in the Block Queue and freezes the corresponding account (mailbox). The Sent Items folder in the mail program (MS Outlook) on the client itself does not contain these messages. The client with the account was checked by different antiviruses. Watched startup and running processes, but everything is clean. Help deal with this problem.