Como de costume, o excelente Wiki do Arch tem a resposta ( link ). Citando textualmente:
E-mail character encoding
When using Mutt there are two levels where the character sets that
must be specified:
- The text editor used to write the e-mail must save it in the desired encoding.
- Mutt will then check the e-mail and determine which encoding is the more appropriate according to the priority you specified in the
send_charset variable. Default: "us-ascii:iso-8859-1:utf-8".
So if you write an e-mail with characters allowed in ISO-8859-1 (like
'résumé'), but without characters specific to Unicode, then Mutt will
set the encoding to ISO-8859-1.
To avoid this behaviour, set the variable in your muttrc:
set send_charset="us-ascii:utf-8"
or even
set send_charset="utf-8"
The first compatible charset starting from the left will be used.
Since UTF-8 is a superset of US-ASCII it does not harm to leave it in
front of UTF-8, it may ensure old MUA will not get confused when
seeing the charset in the e-mail header.
Você pode fazer isso a partir da linha de comando, e não a partir do muttrc, prefixando
-e 'set send_charset="utf-8"'
para os flags de comando.