User cannot register using double period email address

Hi Support

I have two users who both have unique email addresses that include double periods in the address.

For example:

[email protected]
[email protected]

When they register, they're taken to a 404 page, and the user doesn't register in the backend of WordPress. There is no account creation.

Whereas all other email addresses are successful. The only difference we can find is the double "." in the email address. Is there any way of getting around this, or is there something built into the theme or WordPress to prevent this type of registration?

Kind Regards,
Peter Lowen
Constructive Visual