The `h' headers command at the outermost level of the mailer will display the current message ignoring the current setting of the `weed' option. This is most useful for answering questions of the form "I wonder what this guy put in his header?" and such. This command does not show up on the mini-menu because it is somewhat esoteric, but it does appear on the `?' help screen (can you find it there, though?).
The `@' command at the outermost level of the mailer will output a screen of debugging information, including the number of lines and offsets of each of the messages in the current mailbox.
The `#' command at the outermost level of the mailer will display the entire stored `record structure' for the current message.
The `%' command will display the full computed return address of the current message.
Starting up elm with the "-d" debug option will create a file called ELM:debug.info in your home directory and contain a wealth of useful information (to me, at least!) to aid in tracking down what errors are occurring and why.
If there are any problems with the mailer, please try to recreate the error with the debug option enabled and set to the highest level (11) before sending defect reports my way.
One final note: all error names reported by the program are documented in the AT&T System V Interface Definition Reference Manual in errno(2).