Question: Am I allowed to use formmail.pl?
No! Spammers have tools that search sites 24/7 for this script. It's just a matter of time before they find it and start using it to send out vast amounts of spam.
Any form mailer these days should have a captcha requirement to prevent abuse by bots. Google offers ReCaptcha. More information can be found here.
https://www.google.com/recaptcha/intro/index.html