“End of Message” or EOM signifies the end of a message, often an email. The subject of an e-mail may contain such an acronym to signify that all content is in the subject line so that the message itself does not need to be opened (e.g., “No classes Monday (EOM)”). This practice can save the time of the receiver and has been recommended to increase productivity.

EOM can also be used in conjunction with “No Reply Necessary” or NRN to signify that the sender doesn’t require (or would prefer not to receive) a response (e.g., “Campaign has launched (EOM/NRN)”)

via wikipedia