Hello Erica,
please take care not to leave the subject field empty!
In lots of spaces the subject is used to create a link to the posting (i.e. the "New Messages since last visit", search results...).
If there is no subject, there is nothing to click on (although, in the html code, the link is existent, the user still needs something to point the mouse to...).
This way you might loose readers for your message.
Thanks,
MudGuard