I agree with Muralidhar if you are wanting to isolate this to transaction SU01, but just wanted to add that it might be advisable to create a custom variant transaction of the new standard transaction variant in a second step, for administrating users of other types who should not have an SMTP-address...
Which release are you on? As you have recently asked questions about LDAP integration, perhaps an editable but mandatory field is not what you are looking for, but rather a "display only" field which must be populated when the user is created?
If you are using SSO with user provisioning from AD, then you should also try to avoid SU01. For assigning roles, if you do this manually, you can do the same in PFCG. (eg. the user must specify which roles they need, and not just "same access as user XYZ..." etc...)
Just some thoughts incase there is more consideration required behind this, but Muralidhar is correct as per how you have stated your question.
Edited by: Julius Bussche on Mar 27, 2009 9:40 PM