When I go to the website it is requesting me to sign up for a monthly fee.
MXToolbox doesn't actually block any email addresses; instead it's an information system that monitors various email addresses, IPs, and mailboxes for errors and problems in their service. If it's reporting that your address is blocked, then it's likely blocked by another watchdog site, either at an IP or address level.
There's usually no way to manually remove a blacklist from a watchdog, they mostly expire over time, but we'd be happy to dig into the address and see what's going on there. Please fire us a message over Facebook Messenger or Twitter, and we'd be happy to work through it and get you back to normal.
Retrieving data ...