Primary Email Format Restrictions?

We are working on integrating our software with Okta, and have run into an issue with the primary email that we generate for logins.

Logins are unable to use a real login, for various reasons. Our primary email follows the format _ username_@pd. customerName. During testing, emails such as rfoster123@pd.development work great, but rfoster123@pd.SonetoDevelopmentTenant fails on user creation through the API. The error message is login: Username must be in the form of an email address .

How are email addresses validated in the API? Is there a maximum length for the final segment of the domain that we should take into account when constructing the email addresses in a format we can use effectively?