How are they going to receive mail? All blocks of IP's from any provider are blocked, usually huge blocks, larger than /24 often. No one is getting to any comcast users, they as do many others publish lists of their IP ranges so you can block then in your server or use an RBL.
>If your ISP provider is filtering email ports 25 going out you will have to use a SMTP relay. If It blocks port 25 comming in you won't be able to have self-hosted email address. Note that in both cases it won't prevent you from having a fully confidential email address, and that these blockages will be automatically detected and you Own-Mailbox will only propose configurations that will work.
So from that I think that it can use the ISPs email server.
