Create Extension -Parameter [contact.email] value is invalid

When attempting to add a contact using 'Create Extension API', with an email address ending in 'x' (i.e. jsmith@abc.comx', I receive an error:

Parameter [contact.email] value is invalid.

After removing the 'x', it imports fine. The 'x' bypasses sending an email when an extension is assigned so is there some other way to accomplish this via the API or can the validation for the email field be changed to accommodate the '.comx' extension? The 'x' is allowed when entering the email through the web portal.
Thanks!