Role based emails are email addresses like "abuse@" and "postmaster@". Email addresses that are generally used by a system, or a group of users.
We have found that role based are often the cause of high bounce rates and abuse reports and therefore we do block them when you try to import lists of email addresses.
You can still add these email addresses individually, and of course they are not blocked if a user subscribers themselves via a form or other method.
The role based emails we are currently blocking are as follows: