Shared sending domain
A sending domain provided by an ESP for customers to use when they haven’t configured a custom domain; reputation is shared/pooled.
Smishing
Phishing conducted via SMS text messages.
SMTP
Simple Mail Transfer Protocol: the standard protocol used to send and relay email between mail servers.
SMTP Error (Simple Mail Transfer Protocol)
A server response code/message that indicates delivery status or why a message was rejected/temporarily deferred (e.g., 4xx temporary, 5xx permanent).
SNDS (Microsoft Smart Network Data Services)
A Microsoft portal for monitoring Outlook/Hotmail sending IP reputation and related signals, helping diagnose delivery issues.
Soft Bounce
A temporary delivery failure (e.g., mailbox full, server down, rate limiting), often indicated by 4xx SMTP codes. Messages may be retried later.
Spam
Unwanted or unsolicited email. Not all unsolicited email is spam, but spam is typically unwanted and often bulk-sent.
Spam complaint
A recipient action marking a message as spam/junk (or moving it to spam), which negatively impacts sender reputation.
Spam Filter
Rules and systems (including machine learning) used to detect and divert unwanted or suspicious email away from the inbox.
Spam Trap
A monitored address used to identify bad sending practices. Types include pristine, recycled, and typo traps; hits can damage reputation and cause blocklisting.