user.js settings I use:

Setting

Value

Rationale

mailnews.wraplength

0

Disable text wrapping in composer

mail.strictly_mime

true

Use base64/quoted-printable only, do not use 8bit/7bit encoding. From https://forums.zoho.com/topic/zoho-break-dkim-signatures-body-hash-if-8bit-encoding-are-used-through-smtp