FWIW, with Linode, all that is needed is to ask to open the port with reference to MIAB. No questions asked. They support the project even by putting up a guide: How to Create an Email Server with Mail-in-a-Box | Linode
FWIW, with Linode, all that is needed is to ask to open the port with reference to MIAB. No questions asked. They support the project even by putting up a guide: How to Create an Email Server with Mail-in-a-Box | Linode