I used to have the same issue,if I remember correctly it has to do with the way AOL's servers are . Using k9 or another app won't change it. The on.y work around I found (and this is the solution I currently use) is to forward all your AOL email to your gmail account. You can then setup your gmail account to send out emails as you AOL account. Its not too hard to setup initially and you will never have to bother with it again. Then you can use your gmail app to handle everything. Now I don't remember all the setting but I'm sure a quick Google search can take care of it for you.
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