What is SMTP

SMTP is the protocol mail systems use to relay mail between hosts. RFC 5321 defines it. A client opens a connection to port 25 on a mail exchanger, greets the server, and transfers the message with MAIL, RCPT, and DATA commands.

RFC 5321 also defines the envelope: the MAIL command carries the reverse-path, often called the envelope From, and the RCPT commands carry forward-path addresses. That envelope is not the same header block the user sees in a mailbox.

Relays and errors

RFC 5321 assigns three-digit reply codes. A 2xx code means the command succeeded for that stage. A 4xx code is a transient failure; the client may retry later. A 5xx code is a permanent failure for that recipient or transaction.

Many operators also publish enhanced status codes in the form x.y.z after the three-digit code. Google's SMTP error list is one vendor table that maps those codes to plain-language causes. This site quotes that list for Gmail rows.

Full version
RFC 5321 specifies commands, reply codes, and the envelope separate from message content.
MX record
An MX record names the mail exchanger that accepts SMTP mail for the domain. RFC 1035 defines the type. RFC 5321 says the sender looks up the MX set and tries the hosts in preference order.
STARTTLS
STARTTLS is an SMTP extension in RFC 3207. After the client sends EHLO, the server may advertise STARTTLS, and both sides upgrade the connection to TLS before MAIL and RCPT.
Envelope From
The envelope From is the address in the SMTP MAIL FROM command. The header From is the mailbox listed in the message's From header field. RFC 5321 defines the envelope; RFC 5322 defines the message header.