Been there, done that
There are enough very simple distros out there to do this.
This is really a setup task rather than a product.
Just download any linux distro, follow the setup for its apache component and download iredmail, a mix of other open sources apps that installs a functioning mail server in 5 minutes. Its not the best mail server out there and is really just a marketing technique for their other products but it works for small scale mail server.
To be honest, you really should install this stuff manually so that you know how to handle linux and the other software. Otherwise you can end up with massive security holes or major failures.
As for IPv4, its actually pretty simple. You just use dynddns, most ISPs router firmware supports dyn's login making a dynamic IP no problem. You just have to use your ISP to send mail as most mail servers treat mail from dynamic IPs as spam.