The reason you having problems sending to places like hotmail or aol is because you have not implemented spf records. Many hosts are now blocking mail if they do not have spf records in place. It's a great way to reduce spam but it should not be used alone as some of the large providers are doing.
Get some SPF records in place and make sure they are setup properly and you will not have issues sending email to aol anymore unless of course you have other misconfigurations.
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