How to increase email storage limit

Nope. Instead back up your emails and start the long but necessary process of switching to a portable email supplier like Gmail or Outlook. 

There have been so many faults with VM over the past couple of years that you are lucky not to have lost them already.

VM stopped issuing new email accounts over two years ago, and there are frequent rumours that they will close email altogether. VM don't keep email backups, and advise users to do so.

Nothing has been said officially yet, but you will very likely have to move your email one day, so you should start now while you still can.

You should never ever use any email system for ‘archiving’ emails. You should always copy important ones to local storage and ideally also back them up to the cloud.