MH keeps messages until you tell it to delete them. The command for removing messages is rmm. If you don't give message numbers, rmm removes the current message. That's handy when you're reading new mail that you don't want to keep. A typical mail-reading session might look like this:
% inc
...
% show
...
% rmm
% next
...
The command rmm 3 5 would remove messages 3 and
5. MH sequences (like rmm picked) and message ranges (like
rmm last:3) will work, too.
By default, rmm doesn't actually remove a message; it just changes the message filename to "hide" it from MH. Later, a system program should delete the "removed" messages. If you accidentally use rmm, read about Removing and Recovering Messages or ask an MH expert to help you.