How to remove unwanted mails from UNIX mailboxes or folders

We have already documented procedure for removing all emails from POP3 using a shell script.

But what about IMAP based mailboxes? There is cleanup-maildir python script. It It cleans up old messages in FOLDERNAME; the exact action taken depends on COMMAND.

From the article:
Maildir saves each mail in a separate file, it is much easier than it used to be to manipulate the mails. Everyone can write some simple script to do some cleanup based on its needs. A while ago I have stumbled across this python script that does most of the things I needed to cleanup maildir folders.

Now, even if Maildir is much faster for many emails kept on the server, when we reach a huge number of files in a single folder the access times to that folder will be considerably slower. Now I am not talking here about a couple of hundreds of mails, but some huge mailboxes with thousands of mails and huge sizes (over 3-5GB in size). You will be amazed that there are peoples that will do that? They are most probably using IMAP and keeping all their mails on the server, or even POP3 and saving one copy of each mail on the server.