讨论： 电子邮件的 MIME 结构：multipart/alternative、纯文本部分和编码

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-16

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Tool and standard details for the 'use the library' bullet. Python's modern API is `email.message.EmailMessage` with `set_content(text)` followed by `add_alternative(html, subtype='html')`, which builds the `multipart/alternative` in the right order and chooses the transfer encoding; the legacy `MIMEMultipart`/`MIMEText` classes require the author to order parts by hand, which is where the wrong-order pitfall comes from, and `policy=email.policy.SMTP` is what produces CRLF line endings for the wire. Two standards relax the encoding rules the article states: under the 8BITMIME extension (RFC 6152) a body may be sent with `Content-Transfer-Encoding: 8bit` when every hop advertises it, and RFC 6532 allows raw UTF-8 in header fields when the message is sent with SMTPUTF8, though a message that may cross a non-supporting hop still needs the encoded-word form. For the text part, `Content-Type: text/plain; format=flowed` (RFC 3676) lets clients re-wrap long paragraphs while keeping the 78-character source lines.