Emails stuck in Outbox, Exchange Activesync

[AC Question]

We've had a couple users recently who have reported that emails are randomly getting stuck in their Outbox. This is on an Exchange Activesync account that is connecting to our on-premise Exchange 2010 server. All server-side tests show no issue. Most of our user's have no reported problems with sending emails.
I deleted and recreated these user's accounts on their Samsung Galaxy S5, but that did not solve the issue. It seems to have to do with emails that have attachments.