SMTP

SMTP — Simple Mail Transfer Protocol — is the standard that mail servers use to send email across the internet. When people refer to your "primary SMTP address," they mean the main, authoritative email address of record for an account — as distinct from any aliases that also route to the same mailbox.

The distinction matters for naming conventions. The recommendation is to make your primary SMTP address follow a clean, programmatic format — first.last@domain — even if you also keep friendlier aliases. The primary address is the one the system treats as canonical, so it's the one worth getting right from the start.

SMTP is also the layer that email authentication protects: SPF, DKIM, and DMARC all exist to add trust and verification on top of SMTP, which was originally designed without any built-in way to confirm who a message really came from.