I presume by "sending email messages to a test email" you meant an AOL email address of your own, yes?
Unfortunately, once we process an order and the receiving mailserver accepts the outgoing message, we have no control or scrutiny over eventual delivery of that message to the recipient's actual inbox, if it ever even makes it there at all. We do everything possible to help ensure delivery can happen, but there's only so much we can do. I'll ask Development to look for anything new or unusual about our email traffic to AOL addresses.
Overaggressive spam filters are the usual problem with apparent non-delivery only to some recipients, so the first thing we suggest is asking recipients to check their spam folder and mark the message as Not Spam to help retrain the filter. We also recommend keeping any custom thank-you email message as short and to-the-point as possible, with few if any added links/URL or other promotional language that could mistakenly flag the message as potential spam.
We've also seen some email providers often have lengthy delivery delays of an hour or more (Yahoo!Mail in particular), and some users may have restrictive email settings that only accept email from people already in their address book or contact list and send the rest to spam, or even outright delete it, before the recipient ever sees it.