Might want to check the Terms of Use / AUP on your Residential service, because every one I have ever read says you can not host email, and 80% of them say you can host any server of any kind.

Now most of them do not actually enforce it unless you become a problem, but most of them do put active measures on the network to stop SMTP Servers

For example here is a Exerpt from Comcast AUP prohibiting email and web hosting [1]

>>>use or run dedicated, stand-alone equipment or servers from the Premises that provide network content or any other services to anyone outside of your Premises local area network (“Premises LAN”), also commonly referred to as public services or servers. Examples of prohibited equipment and servers include, but are not limited to, email, web hosting, file sharing, and proxy services and servers;

[1] https://www.xfinity.com/corporate/customers/policies/highspe...