You can request a delivery or read receipt, but there's no guarantee the 
recipient will honor the request.  There's no way to guarantee delivery.
-- 
Brian Tillman [MVP-Outlook] 
You can configure Outlook to add a notification header to your 
outbound e-mails to request the recipient's e-mail client send a read 
receipt.  That probably will not work because more e-mail users, and 
maybe including yourself, have disabled sending read receipts.  The 
user installs Outlook and then configures it to ignore all requests 
for read receipts, or they get sufficiently nuisanced with the prompt 
due to the Prompt default setting for read receipts and then go into 
options to disable them.  The recipient dictates whether or not they 
will send you a read receipt.  Unless you are in a company environment 
where policy can dictate that you enable read receipts or answer Yes 
when prompted, don't expect anyone to bother sending them back to you.
You could ask for a delivery reciept but all that tells you is that 
your e-mail got to their e-mail server, not that the user ever got it 
in their mailbox and not if they have read it.  Many, maybe most, mail 
servers will not honor delivery reciepts.  You already get negative 
feedback if your e-mail is not deliverable, so positive feedback is 
superfluous and an extra drain on their resources.
There is an old spammer's trick of inserting web beacons inside of 
HTML-formatted e-mails; however, if the recipient finds out then they 
might thereafter treat you as a spammer, block your e-mails, and 
report you to the blacklists.  Also, web beacons only work if a server 
proffers the hidden image file and most users nowadays configure their 
e-mail (if not already the default) to block linked images.  So that 
old trick doesn't work anymore.
If you want to know if they received your e-mails, ask them to send a 
reply saying so.