Yes this has happened to me , try deleting the Samsung app and install again, this worked for me BUT its not using Imap or POP3 now , so I've deleted it completely and I'm using the outlook app instead. I wasn't sure if my email account had been compromised
Same happened to me last night, getting the same message in my outbox where the messages are stuck. I am having to reply to emails on my tablet can't do it on my phone, had phone almost 3 yrs without this problem before!!