Previous SMTP standard. Introduced port 587 and clarified relay rules. Obsoleted by RFC 5321 in 2008.

Defines the SIZE extension that lets a sending MTA declare message size upfront so the receiver can reject oversized mail before transfer.

Defines the EHLO command and the ESMTP extension framework that all modern SMTP servers use.

Defines the 8BITMIME SMTP extension, allowing 8-bit message content without base64 encoding.

Defines the STARTTLS SMTP extension that upgrades a plain-text SMTP connection to TLS in-band.

Defines the SMTP AUTH command and the SASL framework used to authenticate SMTP clients to submission servers.

Defines the MIME content-type header and the framework for non-ASCII message bodies.

Defines the media type system (text, image, audio, video, application, multipart, message).

Defines encoded-word syntax for non-ASCII characters in headers.

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