Anything without a dot in the domain may not work on the internet today, but ICANN is busy approving a whole stack of new TLDs, many of which will be owned by a single company. They may well want use those new TLDs to set up email addresses like "[email protected]" without any dots in the domain.

Also, some systems may need to send email internally within a local network, in which case it's perfectly legitimate for the local domain to be unqualified.

And as for you never having seen an email address with a space in it.... well, that's lucky for you. They do exist. And so do a bunch of other weird combinations. You can basically have anything you like in front of the @ symbol, and there are people out there who do. I suspect those people find they have a lot of trouble filling in web forms, though.

Validating email addresses easy. Validating them correctly is *hard*. It looks like it ought to be easy, but it isn't. Most of the tutorials on the web telling you how to do it are wrong.