I just tried something similar. 

In the filters, If you have "From specific domains " selected, then i think it is looking only for a domain, not the @ sign.  i tried one with *@ like you have shown, and it did not prefix the subject.  I removed the *@ and just left the domain name, and it added the prefix on the subject.  (this is SM version 14 too)

Hi Eric,

For the content filter condition try changing the Comparison field to "Does Not Match" and put domain.tld in the text field.  Then remove the Sender authenticated condition. For the action, make sure you are using "Add Text to Subject" for the action.  So the content filter should look something like this:

You may also need to upgrade to the latest build, 7125, and if you still cannot get the content filter to work you may need a support ticket.

Emily,

Thanks, I upgraded already and it's working fine. Customer didn't want it, but I got it to work.

What they were actually asking for was the warning messages in the body you can do on Exchange and Office365.

Eric,

I'm glad it's working for you now.  I'll go ahead and mark this as resolved.