Changing the number of displayed lines of the message list
Each message in your mailbox has a one-line heading in the message list. If you have more than 24 messages, the first headings from the message list scroll past the top of your screen. The set screen option controls how many lines of the list are displayed at a time.
To change the number of lines of the message list that are displayed at a time, in your $HOME/.mailrc file, type:
In this example, the system will display 20 message headers at a time. Use either the h or z subcommand to view additional groups of headers. You can also type this subcommand at the mailbox prompt.