Changed primary SMTP for user - can't add AS account with new SMTP

I changed the primary SMTP for a user and am unable to add the active sync account to an iPhone using the new SMTP address. If I add the account using the original default SMTP it works fine. The new primary SMTP IS being used correctly with outbound emails - I just can't add the account using the updated primary SMTP.

Anyway to make that email address acceptable when adding the account to devices?