I get a 550 MI:SPF error, telling me that my IP address does not match the valid sender. In other words, it looks like a spoofed email.
Of course it does – you're trying to use someone else's address in the Envelope-From. That's practically the definition of spoofed.
Given that I'm just running Telnet from my Windows desktop, how do I set up a valid request to validate emails with?
Specify the test address as the recipient, i.e. in
For the sender, instead use an address belonging to a domain that either allows your desktop PC's IP address, or doesn't have SPF restrictions at all. Ideally this should be a domain that you actually have the rights to. (Note that subdomains can have their own SPF records independently from the parent domain.)
Are there any email services that can relay through Telnet
"relay through telnet" is a nonsensical requirement. Mail delivery over SMTP does not involve Telnet at all.
You're just using the
telnet command because it also works as a raw TCP client, but other software and services have no need for this – they make their own TCP connections through the OS, and they directly send the same commands that you're typing in.