Each inbox is limited to 20 messages. The messages do not get deleted automatically when they are read. This is not an email replacement program. It is intended as chat fun mail. Please make make sure to set KILL option ON otherwise others will be able to use your nick.
HELP are the same.
The recipient's nickname must be registered with NickServ. It is also used to send memos to a channel - then the sender need to be at least AOP in the channel. This command sends a memo to SOP's and Founder of the specified channel. If ALL is specified, all of your memos are removed. Memos cannot be deleted unless read.