There are a few reasons that they could get this issue.
1. Are the security groups set up with these parameters?
2. Is the email they are trying to create under a specific Program and does that folder have permissions for an Org Unit? If so, do they have the same permissions? Is the Program folder inside another Program folder and do they have the exact Org Unit access auth?
3. Did you create a custom Email Type and are they using that or the OOTB email type?