Solução: eu instalei o ssmtp + registrei um endereço de e-mail gratuito com ssl smtp da seguinte forma:
# opkg install ssmtp
$ cat /etc/ssmtp/ssmtp.conf
[email protected]
mailhub=SMTP.EXAMPLE.COM:465
rewriteDomain=EXAMPLE.COM
hostname=EXAMPLE.COM
FromLineOverride=YES
UseTLS=YES
AuthUser=MYEMAIL
AuthPass=PASSWORDHERE
$ echo -e "From: [email protected]\nSubject: SUBJECT\nReply-to: [email protected]\n\nBODYHERE" \
> | ssmtp -vvv [email protected]