Good advice there - I would add that one should limit the number of messages in folders - especially the inbox.
I have 74 folders in total - I archive at the end of every year, plus I have a folder structure for various interests and hobbies - e.g. PPRuNe, flying, Porsche 928, games, ebay etc.
The largest number of messages in any one folder is about 7000.
Take the opportunity to back up the *.dbx files from time to time! Preferably before running the compactor.
SD