I ask out of curiosity. I have never seen this exception before! I know I shall not ask any student about their disability. My university is in the Anglosphere.
Using their @universitydomain.edu, a Disability Adviser (from the university's Disability Office) — and the Undergraduate Chair — emailed that one of my students has legitimate basis, and shall be expected to, email from her personal @gmail.com. All her instructors shall not expect her to email from her @universitydomain.edu. But we can still reply to her official @universitydomain.edu, that is set up to forward everything to her Gmail. I know that she's a full time four year undergraduate, not an exchange student.
What can this "legitimate basis" be? What would legitimize someone using their @gmail.com, but hamper them from their @universitydomain.edu?