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If the outgoing mail SMTP mail server is being given a different mail address than the given ISP mail account address then there might be a problem. For example, if the ISP gave you yourname@isp.com but you're using Outlook to send yourname@example.com. Some places prohibit this to prevent abuse of their system.
Another key is that you say this person is connecting to AOL and then sending mail through another server. I know a lot of ISP mail servers require you to actually be logged into their service to be able to send mail. In other words, your client can't use AOL to connect to the net and then use another comapny's mail server. This is of course specific to the mail server. Not all systems are set up this way.