I hoped AOL would tell me my reputation, but it says “undisclosed” 
I believe there is only one recipient (one To header?).
But…I think fixing it by tweaking various things like the www in the Message-ID, Reply-To being equal to the To header, adding a text alternative, and adding doctype and html tags as necessary.
I saw one AOL email go through with these changes, so it might have helped.
Regardless, we’re going to setup Mailgun as a SMTP relay for all our email. I’m sure we won’t have any problems with this in place!