Yes they can! It's their prerogative to allow people to sign up or not with any email address, full stop.

It might make them bad netizens and you may not like it. But the spec doesn't compel any behavior. It's just a way to communicate technical ideas and ideals.

Precisely. This sketch always comes to mind https://youtu.be/hNoS2BU6bbQ

You're under no obligation to accept names that are hard (as in painful) for you (except if you're the police, I suppose. Ironically)

This is paltry nonsense. Gmail and similar host users are used to treating the dots as decorative, and will register with john.smith@ then try to use "Forgot username?" with johnsmith@ instead. They'll end up with three GitHub accounts registered to the same mailbox and be confused as heck about how they're still getting email notifications for an account that they can't recover a password for.

You can't break user expectations and mental models by pointing to the spec as justification. The spec exists to serve users, not the other way around.

Sure, and meanwhile if there are collisions without periods on that domain (randy@somewhere, r.andy@somewhere and rand.y@somewhere, for instance), only one of them gets an account.

In the absence of being able to count on specs, I guess the user should expect a race?

The fact that we just now found out that this is the case, yet would be completely unable to find anyone complaining about it except in hypothetical terms, tells you exactly how important it is.

This stuff just causes your business to mysteriously have low customer satisfaction. You're doing 95% as well as the successful business but for 5% of your customers these "unimportant" problems make it awful to deal with you and they stay away and tell others.

Most of them can't specifically point to the problem, their impression is just that your services don't work properly. They're right.

How would they complain to you? How would they know what the problem is?

Assuming they don't just fail to register and move on ...