Simple message with text in the body of the email
mutt -s "Test from domain" email@example.com < /temp/message.txt
And if you need to send an attachment
mutt -s "Test from domain" firstname.lastname@example.org < /temp/message.txt -a /temp/file.jpg
This is another way to write the test, having the body in the same line.
echo "This is the body" | mutt -s "Testing mutt" email@example.com -a /temp/file.jpg