Domains ending with a dot are valid though, and it's needed sometimes. For example, someone@ai. (ai. is a TLD) is a different email than someone@ai (ai is a local hostname)
When is this needed in the context of a Rails app? You never want to send emails to local hosts, and emails directly on the TLD is possible, but very little will work with them in practice.
That's not entirely true, it depends on how your mailing infrastructure is set up.
That doesn't mean you have to support their use. I have never seen a company support an email ending in a period and the world seems to continue along.