Being attacked by virus spamming

It seems likes more than once a day every day users will get emails with attachments that are caught as viruses and quarantined by our Symantec Antivirus Corporate 7.6. It quarantines the files but still the user receives the email (and with edgy management users-we don’t want that). Recently they are using “undeliverable message receipts” errors – which look authentic but are junk. It makes it look like our server is being used to send out a message to an undeliverable address. What way is there to protect against these spammings if they are from all different email addresses. Another company that I know gets spammed by “adult” emails from hotmail accounts all the time. Is there a way to help against these without having to block every email coming from a particular domain? Thanks!