Notice period for Management trainees - Can we accept email as resignation letter?

Hi , I work as an HR manager for a start up company. One of our employees (management trainee), left the job without serving notice period. The said person sent their resignation citing medical grounds through email . Can we accept the email as resignation letter and I also want to know can the company withhold the salary for not serving the notice period.

What were the terms and conditions for the management trainee?
All depends on conditions which have been mutually accepted by both company and employee/trainee.
If the trainee has left without serving notice period, company is well within rights to set off notice period pay against dues to the trainee.
Company should make a clear policy for trainees.
Make out the offer letter clearly mentioning conditions of work/training and exit methods/procedures.It can even mention that cost of training will be recovered if people quit in between.
Cost should be reasonable, quantifiable and proportionate.