What is Sending domain (lookalike / secondary domain)?
A domain used only for outreach, separate from your primary company domain, so cold email reputation never affects your normal mail.
Cold email carries reputation risk. Sending it from yourcompany.com means a bad campaign can put your CEO's ordinary email in spam. The practice is to register lookalike domains — yourcompany-hq.com, getyourcompany.com — that redirect to the main site, host two or three mailboxes each, authenticate them, warm them, and send outreach from those. If one is burned, retire it.