Email is Received but Cannot Send Email

Exchange users could connect to their mailboxes (OWA and Outlook), but any attempt to send email resulted in the message being stuck in the Drafts folder (when using OWA) and the Outbox (when using Outlook). The Exchange submission service would never see the email and attempt to send it. All users could receive email without any issues. The most common reasons for this type of problems are DNS, IPv6, and firewall. However, all of these were investigated extensively and no issues were found. The issue was resolved by unmounting the Exchange database, restarting the Microsoft Information Store service, and remounting the Exchange database. Afterwards, all email in user mailboxes that was “stuck” was queued for delivery.

Networking Exchange 2013