Edit the configuration file (you may have to install nano "ipkg install nano"):
# Use "tls_certcheck off" if you don't have ca-certificates.crt file.
# Log to a separate file; use with no options to disable this feature
# logfile /opt/logs/msmtp
# Uncomment if you want to log to syslog facility, which is disabled by default
echo -e "Subject: subject \n\nThis is the body" | msmtp firstname.lastname@example.org