It may be infeasible to run a new SMTP-based mail service from "residential IP's" that can interact with the existing email empire, dominated by store and forwa

It may be infeasible to run a new SMTP-based mail service from "residential IP's" that can interact with the existing email empire, dominated by store and forward middlemen who expect to make money from the "free" email service they provide. That empire amounts to a junk email delivery service and later a way to gather information about email users. The later purpose is probably why you want to run a new email service? However it is certainly feasible to run a new SMTP-based email service from residential IP's that does NOT interact with the existing email empire. One with no middlemen. The sender's SMTP server talks directly to the recipient's SMTP server. You decide what port you want to use. There are thousands to choose from. There are multiple ways to do this, but I rarely if ever see this option discussed. I suspect it's because like DNS most users are not comfortable configuring mail servers nor with NAT traversal. If indeed the motivation for running your own mail service is because you do not want your mail stored on third part servers (whether in the sender's mail folders or the recipient's), then the ability to interact with the existing store and forward email providers seems a counterproductive requirement.