Many ISPs (UK anyway) only allow you to use their SMTP server from your ISP connection. Many also stop you from using your own SMTP server (altho I don't understand why). So moving my laptop between home and office is pain - I have to switch my email client's configured smtp server each time.
Or - would be great for mobile email clients, where potentially an app could be installed on any network and IP.
Solution: a paid-for SMTP service, using some kind of authentication mechanism (e.g SMTP-AUTH which most clients can handle) for a nominal monthly/annual fee.
What do you think?
Spam countermeasures, they usually block the port.
If you do follow up with this idea you will also have to find ways to prevent spam. Very troublesome.