Upon further research on email names, I found the standard rule is as stated:
USERNAME(local part)
The username, or local part, can consist of up to 64 characters long and consist of any combination of letters, numbers, and other ASCii 7bit special characters. Periods are acceptable as long as they are not the first or last characters, and they are not consecutive.
Therefore the email in question: "g.simon@domain-name", conforms to the internet standard, and should be acceptable to WikiTree.