Good day to you!
I've never wondered about this problem, so this solution may not be the best one, but it should work:
I assume your using the notify-by-email command when trying to send a mail ... if not, either you understand what I'm trying to say, or could you post your config for sending mails ? (contactgroups, contacts, command ...etc).
You'll just have to modify the notify-by-email command; instead of directly sending an email via /bin/mail, change it so that it calls a script of your own with -more or less- the same arguments;
inside this script, you'll just have to write an if-then-else algorithm which will decide what the title of the mail will be, depending on the severity ... and voila!
I hope it is clear enough ... if not, just ask