Exchange OWA 2013 Emails Stuck in Drafts folders
Like them all, I too faced this issue.
Checked the DNS, group policy, restarted Transport services, checked if any service was in maintenance mode - all failed.
I opened the services mmc and checked the relevant Exchange services. In my case the 'Microsoft Exchange Mailbox Transport Submission' service was not running.
I started the service and emails started flowing again.
Hope this helps!