A secondary domain (or sending domain) is a domain bought only to send cold email, separate from the domain your company uses every day. If yourcompany.com is your main domain, a secondary one might be getyourcompany.com or yourcompanyhq.com.
Why cold emailers use them
Your main domain carries invoices, customer replies and password resets. Cold email always carries some risk to reputation. Sending it from separate domains means a campaign that goes wrong can't push your everyday email into spam.
How to set one up
- Choose a name close to your brand, ideally
.com, with no hyphens or numbers. - Point the website at your main site, so anyone who visits finds you.
- Add real mailboxes (around three per domain), SPF, DKIM, DMARC and MX.
- Buy it a few weeks before you need it, and warm up every mailbox.
Being honest about it
A secondary domain is still your company. Use real names and a real way to reply or opt out; it protects your main domain, it isn't a way to hide who's sending.