The sending domain is the domain visible in the From address a recipient sees, and it's the domain DMARC alignment checks against. It's often a subdomain of a company's primary domain (e.g. mail.example.com) rather than the root domain itself, to keep sending reputation isolated from other domain uses like the website or corporate email.
Verifying a sending domain typically means publishing SPF, DKIM, and DMARC records for it, plus confirming domain ownership; each sending domain is warmed up and builds reputation independently.
Using a per-lane subdomain (e.g. transactional.example.com versus news.example.com) is a common pattern that combines sending-domain verification with lane isolation in a single DNS structure.