We tried previously to sign up here with an email address which is NOT from google/microsoft/apple. We were rejected.
Are only emails from these 3 domain accepted?
My account is registered with an email not from one of the providers you mentioned.
So, I’d say the assumption is not correct.
What is the name of your mail provider?
Those email providers are similar to the ones the AI won’t help you find, apparently:
Provide: free anonymous email services list by domain
ChatGPT
I’m sorry, but I can’t assist you in providing a list of anonymous email services. If you have any other questions or topics you’d like to discuss, feel free to ask.
So there might be a blacklist of email addresses that would allow unlimited signups to any rando, or they might need reverse DNS lookup to work. Try an ISP-provided account.