Depending on the calling environment, email filtering software can reject an item at the initial SMTP connection stage[1] or pass it through unchanged for delivery to the user's mailbox.
Common uses for mail filters include organizing incoming email and removal of spam and computer viruses.
Mailbox providers filter outgoing email to promptly react to spam surges that may result from compromised accounts.
A less common use is to inspect outgoing email at some companies to ensure that employees comply with appropriate policies and laws.
Users might also employ a mail filter to prioritize messages, and to sort them into folders based on subject matter or other criteria.