RFC 5322 / START HERE
Email message format, before the details
Internet email message format
This RFC describes the structure of an email message: fields such as From and Subject followed by a body. The delivery envelope is a separate concept.
The idea in one picture
A person writes a message
Headers describe the letter
Mail software reads its structure
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Mail software needs a shared format for composing, reading, and forwarding messages across different systems.
A familiar example
An email app extracts a subject and sender from the message headers before showing the message body.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Message anatomyHeader fields and an optional body are separated by an empty line.→
- Folding long fieldsOne logical header field can span physical lines at permitted folding white space.→
- Mailboxes and groupsA display name is separate from an address; groups collect mailbox addresses.→
- Authors, senders, and recipientsFrom describes authorship, Sender the transmitting agent, and Reply-To reply destinations.→
- Message IDs and reply threadsEach new message has its own ID; reply fields describe relationships to earlier messages.→
- Trace, resending, and trustTrace fields record handling; Resent fields describe a new sending event without changing original authorship.→
This introduction explains RFC 5322 (2008). Read the original RFC ↗