We have setup exchange server 2013.We tried configuring some user mailboxes and room mailboxes.After 3-4 days of using,we encounter Microsoft.Exchange.Data.Storage.MailboxOfflineException in event viewer and after that whenver we try to send mail from user mailboxes ,it will stuck in drafts folder.Could you help us in resolving this issue.I have inserted the screenshot for reference.

Thanks and Regards,

Hari prasad Tammineni

Hi Hari,

Are your DCs running fine. Are you able to login OWA and ECP.

Microsoft Exchange.Data Storage Mailbox Unavailable Exception