Alias: A unique and usually shorter URL (link) that can be distinguished from other links even if they ultimately go to the same Web page. This makes it possible to track which message led viewers to click on the link.

Attachment: An audio, video or other data file that is attached to an email message.

Autoresponder: A computer program that automatically responds with a prewritten message to anyone who sends an email message to a particular email address or uses an online feedback form.

Authentication: A term that refers to standards, such as Sender ID, SPF and DomainKeys/DKIM, that serve to identify that an email is really sent from the domain name and individual listed as the sender. Authentication standards are used to fight spam and spoofing.

Bayesian filter: A spam filter that applies statistics to email message content to determine the probability that it is spam. Bayesian filters are adaptable and can learn to identify new patterns of spam by analyzing incoming email.

Bcc: Blind Carbon Copy -  When a message is blind carbon copied, neither the main recipient nor the Bcc'd recipients can see the addresses in the "Bcc:" field.

Blacklist: A list containing email addresses or IP addresses of suspected spammers. Blacklists are sometimes used to reject incoming mail at the server level before the email reaches the recipient.

Block: An action by an Internet Service Provider to prevent email messages from being forwarded to the end recipient.

Bounces: Email messages that fail to reach their intended destination. "Hard" bounces are caused by invalid email addresses, whereas "soft" bounces are due to temporary conditions, such as overloaded inboxes.