Secondary sending domain
A separate domain, often resembling the brand's main one (example-mail.com for example.com), that cold emailers send from so complaints and filtering fall on it instead.
What it means
Cold email senders often register secondary domains close to their brand and send from mailboxes there, so complaints and filtering fall on those domains rather than the main one.
Industry bodies discourage it. M3AAWG's sender practices recommend using a subdomain rather than a cousin domain to separate parts of a business, warn that cousin domains teach recipients to trust lookalikes and widen the surface for phishing, and say a cousin domain should not be used to get mail delivered when the main domain has reputation problems, which is what spammers do. Its cold email position lists lookalike domains among deceptive delivery methods.
Secondary domains still need full authentication and their own warm-up.
Related terms
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Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes