Is there any way to change the timestamp on an email you send to make
it appear to the person who receives it that it was sent at a
different time? I am specifically interested in changing to an earlier
time, but curious about later as well.

I am looking for effective techniques, suggestions or ideas about how
I might be able to do this. The important thing here is how it appears
in the recipient's email, it does not matter what time it says it was
sent in my email, or what gets recorded on my side.

I've tried changing the time/date on my computer, which obviously didn't work.

Depending on why you are doing this, it could contitute fraud.  You
could be arrested or fined.

i'm not an email expert, but i'm guessing it is timestamped on the
mail server.  You would probably need to install your own mailserver
(exchange, sendmail etc), change the times on it and route your mail
through it.

You need to change your mail server's time, 
mails are timestampted according to your mail server,
mail header will be mail server's time, IP etc...

Back when I was in college, I remember very clearly one of my
classmates did just that thing for his own unscrupulous whims...I
thought he was pretty clever, and he never actually got caught.  After
all, who can argue with a timestamp, right?

In MS Outlook, the emails are stamped based on the time of your local
computer.  So, change the time on your computer and you'll most likely
be OK.  Just make sure you're not connected to an internet time server
of some sort...This is only true if you're using POP/SMTP access for
your email.  If your email network uses a MS Exchange Server, all bets
are off.

Anyways, back to my classmate -- he would sometimes not go in to work
because he would rather sleep in.  He'd wake up way later than when
his job started, do his time changing thing, and send an email to work
telling them that he wouldn't be able to make it in...Eventually, they
told him that his emails weren't ever getting through in time probably
due to a "server lag", so they asked him to call in and leave a
message instead of using email...

There were other things like turning in papers late (with the correct
timestamp of course) and so on, but we won't go into those...

Don't use it for ill...it is fraud and it could get you into big
trouble!  If it's for a late birthday/anniversary, though...well, that
may be a different story...