mailx has two help commands that display lists of commands and functions. When in command mode, you can type ? at the mailx prompt (&) to see a list of commands used in that mode. Likewise, when in input mode (for example, when writing a letter), you can give the equivalent command, ~? to view a list of tilde commands (also called "tilde escapes").
The man pages contain extensive information on mailx in more technical form. To see this entry, give the command:
$ man mailx
or refer to the man Pages(1): User Commands.