This is a note for my future self: When writing an email with only “charset=US-ASCII”, alpine creates an email with:
Content-Type: TEXT/PLAIN; format=flowed; charset=US-ASCII
and everything is fine.
In case of UTF-8 characters inside the text, alpine creates something like:
Content-Type: MULTIPART/MIXED; BOUNDARY="705298698-1667814148-1432049085=:28313"
and the only available part contains:
Content-Type: TEXT/PLAIN; format=flowed; charset=UTF-8
Google tells me that the reason for this is:
Alpine uses a single part MULTIPART/MIXED to apply a protection wrapper around QUOTED-PRINTABLE and BASE64 content to prevent it from being corrupted by various mail delivery systems that append little (typically advertising) things at the end of the message.
Ok, this behavior might come from bad experiences and it seems to work most of the time. Unfortunately if one sends a signed email to a Debian list that checks whether the signature is valid (like for example debian-lts-announce), such an email will be rejected with:
Failed to understand the email or find a signature: UDFormatError:
Cannot handle multipart messages not of type multipart/signed