Emails can be a pain to trouble shoot.
Where I start off is by getting a time, date and to and from address from the user who reports the problem (unless they are reporting a general problem like an error message that shows up on the screen). Check through log files and if the email succesfully left the server then it will say succesful. Logfiles should give the reason. The location on a Windows server is either your log files directory for the mail software, or if using Windows components it's at c:\windows\system32\logfiles\ and you will see a folder for SMTP. Just make sure you have IIS logging the emails correctly. I think by default logging might be off.
I then trace that to which mail server they used and then check the IP at
www.dnsstuff.com and see if the IP is listed in any blacklists. If so then I follow there and see why.
If you have problems with AOL.com then you might not be on their
whitelist. AOL use a blocking technique that if your mail server sends too many emails with in an allocated time (like per hour) then they start to reject emails. This isnt a problem for smaller mail servers, but if you do find it's a problem then register for whitelist status which when you do you tell them you will follow their guidelines which are basics such as not spamming, having correct RDNS entries on the IP etc...
It might be worth checking through the postmaster mail box to see if any emails have bounced back and for what reason.
Another thing to set up will be an SPF record which is sometimes required by hosts. Currently most seem to do a soft fail when an SPF record is not present for a domain. An SPF record is basically a TXT DNS record which tells mail server what IP addresses are authorised to email from whatever.com. To create an SPF record visit
http://www.openspf.org/ and follow the instructions there. If you are referring to the domain I think you are then there is no SPF record set up yet for that domain.
I will try think of other methods to track issues. The above information covers quite a bit though.
