One possiblity is that AOL is treating some of your emails as spam. You could ask the AOL account user to try the following.
Go To Keyword, type in spam, in the window that opens tick the "Blocked Email should be delivered to the spam folder" box (rather than being permanently deleted).
To see the spam folder he, or she, needs to click on Read. That opens up the email page and the number of items in the spam folder is shown bottom left.
If an email then appears in that folder, he, or she, can click on the spam folder to bring up details of the email. If its from you he, or she, can then click on the box "This is not spam." AOL should then allow your future emails.
If the account user already has it set up this way then just ignore everything I've written.