Marketo not capturing form fills of those who have email addresses where the prefix contains numbers only.

I have confirmed that individuals have completed MKTO forms on an MKTO LP and a web page, but there is no record of their existence in MKTO afterward (let alone that they completed the form). The common denominator is that the email address prefixes are all numbers (e.g., 2300000@domain.com or 09456@domain.com).

I've tried manually adding one of the individuals as a new MKTO record, but it works only if I save the record first using a non-all-digit email address as a placeholder and then replace it. 

We do not have any active Global Form Validation Rules in place. 

Anyone else have this issue and know how to resolve? Thanks.