The limit is there to prevent spamming, as users might mail constantly and spam or cause disturbance or inconveniences. The limit is different for different members. People of high positions like Priest/ess or Mods have a higher capacity of sending mails than normal users, because they may have important discussions or notices to mail. The limit is calculated on mails per hour I guess.
If you have reached your limit, then you must wait for an hour for it to reset. You must refrain from sending mails during that period or it might take longer time to reset. It auto automatically gets reset. If not, even after waiting several hours, it might be that you're gagged or its a glitch, not sure.