What is the Unicode guidance regarding the use of a BOM as a UTF-8 encoding signature?

[Julian Bradfield]

Just to digress a little, I get quite a lot of mail which has
BOM/ZWNBSP scattered through it, sometimes at the beginning of the
mail, sometimes at the beginning of the quoted mail to which it is a
reply. Occasionally at the start of every line. Mostly it emanates
from known useless webmail providers such as Yahoo, but some slightly
more reputable providers do it as well. (I don't easily have a list,
as I now filter it out before it hits my mailbox.)

Does anybody have an idea why they do this? Some accident of legacy
coding?