156: Why doesn't Emacs expand my aliases when sending mail?
  
  * You must separate multiple addresses in the headers of the mail buffer
    with commas.  This is because Emacs supports RFC822 standard addresses
    like this one:
  
      To: Willy Smith <wks@xpnsv.lwyrs.com>
  
    However, you do not need to separate addresses with commas in your
    .mailrc file.
  
    WARNING: Emacs breaks up aliases in the .mailrc file into multiple
    addresses both on commas and on whitespace, regardless of any use of
    quotes.  This is probably a bug.  You can get around this by directly
    setting the value of mail-aliases.
  
  * Emacs normally only reads the `.mailrc' file once per session, when you
    start to compose your first mail message.  If you edit .mailrc, you can
    type "M-ESC (build-mail-aliases) RET" to make Emacs reread .mailrc.
    (You have to include the parentheses where they are shown!)
  
  * Emacs does not interpret vendor-specific additions to the format of the
    .mailrc file such as the `source' command.  It also ignores any `set'
    commands.  The only commands it looks at are `alias' and `group'
    commands.