Signature file: A short text file that email users can automatically append at the end of each message they send. Commonly, signature files list the user's name, phone number, company, company URL, etc.

SMTP: Simple Mail Transfer Protocol – A protocol used to send email on the Internet. SMTP is a set of rules regarding the interaction between a program sending email and a program receiving email.

Snail mail: Traditional or surface mail sent through postal services such as the USPS.

Sniffing: A method of determining whether email recipients are capable of receiving HTML-formatted messages. This procedure is not recommended as it is flawed and may result in inaccurate findings.

Soft bounces: Email messages that cannot be delivered to the recipient because of a temporary error, such as a full mailbox.

Spam: (Also known as unsolicited commercial email) – Unwanted, unsolicited junk email sent to a large number of recipients.

SPF: Sender Policy Framework – An authentication protocol used by recipient sites to verify that the originating IP address is authorized to send email for the domain name declared in the "MAIL FROM" line of the mail envelope. SPF is used to identify messages with forged "MAIL FROM" addresses.

Spoofing: The disreputable and often illegal act of falsifying the sender email address to make it appear as if an email message came from somewhere else.

Subject line: The part of an email message where senders can type what the email message is about. Subject lines are considered important by email marketers because they can often influence whether a recipient will open an email message.

Targeting: Using demographics and related information in a customer database to select the most appropriate recipients for a specific email campaign.

Tracking: In an email marketing campaign, measuring behavioral activities such as click-throughs and open-ups.