University portal says references not suitable

So, I am an international student from India, and I've applied through UCAS and it's past application deadline and universities have been reaching out for further documentation and details through email and their portals. I got a message on my intray in Uni of Kent's portal today that my references provided is unsuitable. This is what it says:

The reference submitted is unsuitable. Please ensure that the email provided for your referee is from a verifiable source and not from hotmail, yahoo, gmail etc. account. A reference on headed university or company paper will be acceptable and can be uploaded as a separate document via your portal. Please note, originals will be required at registration of this document.

My teacher submitted through an independent personal email (gmail i think), and I've checked, and nowhere in UCAS guidelines does it say that the referees cannot do this, and in fact UCAS even gives example of referee's email as a yahoo email.

It is the middle of my final board examinations and I'm not sure my school can provide a documentation as Kent is asking for within 12 March, as it is not a common practice to hand out references or predicted grades either.

What should I do? Please reply fast.

Does your teacher have a school / college email address? If so, have them use that address to send their reference directly to the university. (Make sure they include your UCAS id in the subject line, so the uni can match it to your application easily.)

Just to add that if the referee can't provide it via an academic address by the deadline given, just advise the uni that you are working to provide it.

Often they just need to know that you have seen their request and are trying to get a verified version of the document.