Negotiate chunking before sending chunks
01SMTP means Simple Mail Transfer Protocol; MIME means Multipurpose Internet Mail Extensions. CHUNKING advertises an exact-length alternative to SMTP’s DATA transfer. BINARYMIME is a separate capability.
MAIL and RCPT replies are collected and processed. This example has one accepted recipient and a chosen 7-bit message, so BDAT may begin without needing BINARYMIME.
Source for this drawing: RFC 3030 · Read RFC section 2 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
RFC 3030 obsoletes the experimental publication and revises examples. It defines extensions, not a new SMTP version.
Read the updated RFC section ↗Use BDAT only after the receiver advertises CHUNKING in a successful EHLO response. A command line is followed immediately by its bytes, so an unaware receiver could mistake data for commands.
- Use BDAT only after the receiver advertises CHUNKING in a successful EHLO response. A command line is followed immediately by its bytes, so an unaware receiver could mistake data for commands.
- CHUNKING alone does not advertise BINARYMIME or PIPELINING. BINARYMIME can only be used with CHUNKING; PIPELINING must be separately advertised before using it.
- Collect and process all MAIL and RCPT replies before sending message data through a series of BDAT commands. An established connection and at least one accepted recipient are assumed in later scenes.
- Servers offering BDAT MUST continue to support DATA. Appropriately encoded mail can still use DATA; do not send raw binary to an incapable peer.
Fictional example.com mail peers. Counts and byte excerpts are illustrative, not a full MIME message or a deployment configuration. Separate comparisons reset state. This companion is not an SMTP parser or implementation.
Read the RFC section ↗