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Technically, a trailing dot is required for the root domain.

"ai" is not valid but "ai." is.

Though I suspect a lot of software is quite lenient.

Conversely, the address joe@example.com. (with a trailing dot) is valid, but probably breaks countless programs.

Once upon a time it used to work for Youtube, but break the adverts. Halcyon days.

It seems like most MTAs just append it for you quietly, as someone who has chased ghosts through way too many smtp logs.