Section 12.6 of the current debian policy says:

    If your package needs to know what hostname to use on (for
    example) outgoing news and mail messages which are generated
    locally, you should use the file `/etc/mailname'.  It will contain
    the portion after the username and `@' (at) sign for email
    addresses of users on the machine (followed by a newline).

    Such package should check for the existence of this file when it
    is being configured.  If it exists, it should be used without
    comment, although an MTA's configuration script may wish to prompt
    the user even if it finds that this file exists.  If the file does
    not exist, the package should prompt the user for the value
    (preferably using `debconf') and store it in `/etc/mailname' as
    well as using it in the package's configuration.

emacsen-common uses /etc/mailname in
/etc/emacs/site-start.d/00debian-vars.el but does not attempt to
create it if it isn't there, which looks like a policy violation.