The SMTP command used to identify the envelope sender of an email message is MAIL FROM.
MAIL FROM begins an SMTP mail transaction and supplies the reverse-path, commonly called the envelope sender or return address. This address is used for delivery failures and bounce messages. A typical transaction starts with MAIL FROM, continues with one or more RCPT TO commands for recipients, and then uses DATA to transmit the message content.
The envelope is separate from the visible message headers. The address in MAIL FROM can differ from the From: header that users see in an email application. That distinction is important for mailing lists, automated services, forwarding systems, and bounce handling. After final delivery, a receiving server may record the envelope sender in a Return-Path: header.
HELO, or its extended form EHLO, introduces the client and identifies the SMTP session’s sending host. RCPT TO identifies each envelope recipient. DATA marks the beginning of the message header and body. These commands are therefore related, but only MAIL FROM establishes the transaction’s sender or reverse-path.