A primitive mailx emulator using bash, curl and Mailgun

mailx.sh

#!/bin/bash
set -eu -o pipefail

cmd="curl -s --user 'api:$MAILGUN_API_KEY' https://api.mailgun.net/v2/$MAILGUN_DOMAIN/messages"

while getopts "r:c:b:s:a:E" opt; do
  case $opt in
    r)
      cmd+=" -F from='$OPTARG'"
      ;;
    c)
      cmd+=" -F cc='$OPTARG'"
      ;;
    b)
      cmd+=" -F bcc='$OPTARG'"
      ;;
    s)
      cmd+=" -F subject='$OPTARG'"
      ;;
    a)
      cmd+=" -F attachment=@$OPTARG"
      ;;
    E)
      E=1
      ;;
  esac
done

shift $((OPTIND-1))

while (( $# )); do
  cmd+=" -F to='$1'"
  shift
done

stdin=$(</dev/stdin)

[[ -z ${E-} ]] && [[ -z $stdin ]] && exit

cmd+=" -F text='$stdin'"

eval $cmd >/dev/null

e.g. Send via API example:

echo 'Testing some Mailgun awesomeness!' |
mailx.sh -r 'Excited User <mailgun@YOUR_DOMAIN_NAME>' -s 'Hello' YOU@YOUR_DOMAIN_NAME bar@example.com