Open-relay: Open-relay is the third-party relaying of email messages though a mail server. Spammers looking to obscure or hide the source of large volume mailings often use mail servers with open-relay vulnerabilities to deliver their email messages.

Open-up tracking: The process of tracking how many recipients opened their email messages as part of an email marketing campaign. Open-up tracking is only possible using HTML mail.

Open-up rate: The percentage of recipients who opened their email messages. The open-up rate is often used to measure the success of an email marketing campaign.

Opt-in: An approach to email lists in which subscribers must explicitly request to be included in an email campaign or newsletter.

Opt-out: An approach to email lists in which subscribers are included in email campaigns or newsletters until they specifically request not to be subscribed any longer. This method is not recommended and may in some cases be illegal.

Out-of-office replies: Automatic email reply messages triggered by incoming email to a user's inbox, typically activated when users are on vacation or otherwise unavailable through email for an extended period.

Pass-along: An email message that gets forwarded by a subscriber to another person who is not subscribed to the list (See also "Viral Marketing").

Personalization: The insertion of personal greetings in email messages (for instance "Dear John" rather than the generic "Dear Customer"). Personalization requires email list management software that allows for so called mail-merge operations.

Plain text: Text in an email message that contains no formatting elements.

POP: Post Office Protocol – A protocol used to retrieve email from a mail server. Most email clients use either the POP or the newer IMAP protocol.

Post: Send to a distribution list or Usenet newsgroup, i.e. to a quasi-stable group of people.

PUP: Potentially Unwanted Program - A PUP (potentially unwanted program) is a program that may be unwanted, despite the possibility that users consented to download it. PUPs include spyware, adware, and dialers, and are often downloaded in conjunction with a program that the user wants.