I've had many cases where an admin will put semicolons to separate email addresses, within the To: email notifications. There is nothing on that screen to indicate that commas must be used. It is also not caught by validation. Instead, an error appears when a user tries to submit the form and it does not go through. The error does not explain the issue, it only provides the reference code for IT support to look up.
This is a bad user experience, and causes delays and frustration for the user.
Outlook and other programs use semicolons to separate email addresses; so it is a natural assumption to use semicolons instead of commas.