I can't register on website XYZ using my 4d2.org email - it says it's invalid.

Does it say your address is invalid the moment you click the submit button on the registration form, or even while you're still filling out the form? If so, this is almost always the fault of the website in question. There are two common causes:

1. Some email address validation routines are broken and refuse to accept any domain beginning with a number. Domains beginning with numbers have been valid since RFC 1123 in 1989, but since they're still not hugely popular, website operators sometimes don't notice the problem until they're informed of it. If you can replace "4d2.org" with "4dqueuwihbcewquc.com" and get the same error, consider contacting the website owner and letting them know their code is broken.
2. A few even worse email address validation routines won't accept any domain name with fewer than 4 characters. This is much less common today than it used to be, since the explosion of gTLDs like .social, .online, .shop and .xyz has come with a corresponding explosion of short domain names. If "zz9.org" gives the same error as "4d2.org", this is the likely cause, and you should tell the website owner their code is broken.

If you're able to submit the registration form successfully, but you're expecting a verification email message that you never receive, there may be a different issue. You can follow these steps to troubleshoot:

1. Send an email to your 4d2.org address from a different email account.
2. If you don't receive the message, send us a ticket including as many details of the test message you sent as possible (from address, exact date/time, etc.).
3. If you do receive the message, contact the owner of the website where you're trying to register. They may have a code problem.