Why do some user accounts import without email address?

I have a AD UPS sync Setup within SharePoint 2013. The mail to workemail mapping is correct, and the majority of users import with all attributes including email address - However a range of users import without an email address, which definitely exists within AD. All users are within the same AD container.

I cannot find any discrepancies in accounts for those users who do and don't have emails post-UPS sync.

Do you have any suggestions on a possible cause or items to investigate please?