How do I find the worm

My mailbox is filling with what looks like W32.SWEN Some with attachments that are being blocked by the CA virus scanner on my mail server. Others with just a message saying undeliverable message. I am administering a small business network where I receive the administrator mail. I seem to be the only user getting the email. I’m getting about 100 messages a day. I have CA Innoculan on all PC’s in the network but would have to check each one to be sure. Is the worm inside my network or outside? Why am I the only one getting the mail? Help me track down the source to get the points.