https://mailutils.org/
https://unix.stackexchange.com/questions/695987/how-can-i-make-gnu-mailutils-to-use-an-external-smtp-server
## Internal echo "[message]" | mail -s "[subject]" [address] echo "[message]" | mail -E "set verbose" -s "[subject]" [address]
## External SMTP Server echo "[message]" | mail -E "set sendmail=smtps://[server]" -a "Date: `TZ=Asia/Tokyo date -R`" -a "From: [sender]" -s "[subject]" [address] echo "[message]" | mail -E "set verbose" -E "set sendmail=smtps://[server]" -a "Date: `TZ=Asia/Tokyo date -R`" -a "From: [sender]" -s "[subject]" [address]