I had a similar problem. I had 2 AOL accounts setup in ipad's preloaded MAIL app. It was nice to switch accounts without having to log out of either. But soon found it confusing and frustrating that emails sent thru iPad MAIL did not synch or show when I checked AOL from other computers. I stopped using the MAIL app and now check email via safari. I had also changed my passwords. Here is where a problem started that I was unaware of for months. I just happened to check MAIL and I had something like 18 emails in my outbox! Going back to April, with the error message "...server does not allow relaying..." It turned out only emails I had sent directly from either Craigslist or links I had emailed my self thru Stumbleupon where affected, I assume any reply or forwarding from any website would be affected. First I thought It had to do with some policy change about sending clickable links. Actually I had forgot to...you guessed it...update my passwords in iPads setting menu. As any email generated outside of an actual mail program is defaulted to MAIL they were not sent. The pisser is there is no indication anything is wrong until emails aren't received. No popup. No push. Both Craigslist and Stumbleupon reported sent successfully. While unbeknownst to me MAIL was actually handling the task in the background.
Long story short: remember to update your passwords in settings, mail-contacts-calendar, accounts.
