I've only heard of this problem for one client so far, but it's straightforward enough:

San Bernardino County Community College asks for a letter of reference specifically from faculty and would like a way to automatically guard against the students' entering reference email addresses under other domains (e.g. others aside from @sbccd.cc.ca.us). Right now, we have no way to accommodate this, so SBCCD is having to manually check periodically under the Users >> References grid to find any references named by students who are not actually current faculty.

A possible solution might be:

When a system admin sets a question type to "Reference Request" then we could add several new fields to be filled in optionally:

• Domain validation -- e.g. "If reference emails must be from a specific domain, please list those domains"
  [since there may be multiple valid domains for an institution, I would suggest we allow multiple submissions here]
• Validation message -- e.g. "Enter the message that you would like to show to your applicants if they submit a reference email that is not in one of your valid domains"

... and ... that might take care of it.