Negotiate ESMTP extensions

Extended SMTP, or ESMTP, uses the EHLO reply to advertise optional capabilities.

SIZE declares a message-size limit. 8BITMIME supports an extended body format. SMTPUTF8 permits internationalized addresses and headers when both sides agree. PIPELINING can reduce command round trips.

An extension is usable only when the server advertises it. A client must have a fallback or report that the message cannot be sent as constructed.

Capabilities change what the client may put on the wire:

S: 250-SIZE 10485760
S: 250-8BITMIME
S: 250 SMTPUTF8
C: MAIL FROM:<josé@example.org> SMTPUTF8 SIZE=2381

Without SMTPUTF8, the non-ASCII mailbox cannot be sent in that transaction. Without 8BITMIME, a client must encode message bodies into a compatible representation or choose another route. SIZE lets the client avoid sending content the server has already said is too large.

PIPELINING changes timing, not command semantics. The client may send allowed commands without waiting for each reply, but it must still match replies in order and stop correctly after a failure. It must not pipeline commands the extension forbids.

Extensions are hop-by-hop. Support on the submission server does not prove the next relay supports the same feature. The sending system must choose routes and transformations that preserve the message or fail visibly.

Remove SMTPUTF8 from the capability list above. Explain which part of the transaction becomes invalid and why changing only the visible From header would not fix the envelope address.