Check your mail logs for the error codes you get back from AOL. AOL's system does a variety of delays and other 'tatics'. If you can get on their whitelist (a whole other annoyance) you can get more reliable mail to them.
If users tag your messages as spam.. they can block you all together. Start here
http://postmaster.aol.com/
Get a feedback loop.. and then apply to get whitelisted.