Delay message only once

I send messages out to say that a file has been revised, but it may take 30
minutes to update on the server, so I want to delay the message by 30 mins.
If I use Options, do not send before, I have to pick a time by hand - not
automated.
I always use the same email to send - I add into the subject and body the
details of the particular doc/s.
I assigned a category to this template email, which I forward (I haven't
figured out email templates properly - it just sits in a folder and I hit
forward).
I made a rule to delay if the email being sent has this category.
If I later want to forward that already sent email to another person, it
will still have that category, so it will delay again, but I only want it to
delay that first time.
Is there a way to strip off that category once sent?
Or another solution?