Add API endpoint for accepting email
Some hosts, such as GoDaddy, block all SMTP ports, or don't allow using an SMTP server other than their own. A lot of mail services, such as Mandrill, Mailgun, Sparkpost, etc. provide both an SMTP server and a REST API for accepting email. Providing a REST API endpoint for accepting email would allow users on such restrictive hosts to be able to use mailtrap, even when they can't use the SMTP server.
You can use relayhost configuration on another SMTP server or just use email address feature from Bee-Trap plan.
GoDaddy requires their hosted servers to send through their own SMTP relays. "Your server must send mail using our relay servers." (https://www.godaddy.com/help/find-your-email-relay-server-122).
Because of this, there is currently no way to use mailtrap on GoDaddy host servers (without, of course, upgrading to the Bee-Trap plan, as you mentioned).