A mail-transfer transaction
An SMTP transaction supplies a return path with MAIL, one or more recipients with RCPT, and message data through DATA. Replies distinguish acceptance from temporary or permanent failure.
These addresses belong to the transport envelope, not necessarily the displayed From and To fields. Blind copies, forwarding and failure notifications make that distinction important.
DATA framing distinguishes content from its terminator. Basic SMTP ends message data with a line containing only a period. A content line beginning with a period is sent with one extra leading period, which the receiver removes under the transparency rules. This dot-stuffing prevents content from being mistaken for early termination rather than permanently editing punctuation. It describes DATA; other transfer extensions cannot automatically be assigned identical framing.
Submission and relay
A user agent submitting new mail and servers relaying existing mail perform different roles. RFC 6409 separates submission so authentication, local policy and completion of missing fields have an explicit place.
Servers can queue messages for later transfer or retry after temporary problems. An SMTP success reply must be interpreted at its own stage, rather than substituted for final delivery or reading confirmation.
Transport protection and content
SMTP specifies mail transfer rather than automatically providing end-to-end encryption for every hop and stored copy. Submission and access services can protect connections with TLS, including explicitly defined implicit or upgrade modes.
Protecting a connection does not hide a message from every processing server. Account permissions, abuse handling, content signatures and end-to-end protection are separate mechanisms.