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.