Why doesn’t Emacs expand my aliases when sending mail?

See The Emacs Manual in The Emacs Manual.

• Normally, Emacs expands aliases when you send the message. To expand them before this, use M-x expand-mail-aliases.

• Emacs normally only reads the .mailrc file once per session, when you start to compose your first mail message. If you edit the file after this, you can use M-x build-mail-aliases to make Emacs reread it.

• If you like, you can expand mail aliases as abbrevs, as soon as you type them in. To enable this feature, execute the following:

  (add-hook 'mail-mode-hook 'mail-abbrevs-setup)

  Note that the aliases are expanded automatically only after you type a word-separator character (e.g., RET or ,). You can force their expansion by moving point to the end of the alias and typing C-x a e (M-x expand-abbrev).