Exchange not delivering - messages stuck in outbox

Exchange 2016 stopped delivering email mid-day today. Messages sit in the Outbox (same problem via Outlook or OWA) and do not send. No error message is given, they just never leave the outbox. This applies to both internet and external recipients -- that is to say, Outlook won't even deliver email to other users on the same server. What should I be doing to troubleshoot this?
AaronSSHIT ConsultantAsked:
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Are all the Exchange services running?
Valentina PerezExchange ServersCommented:
Hi Aaron,

Could you verify if the databases are mounted:

Get-MailboxDatabaseCopyStatus *

AaronSSHIT ConsultantAuthor Commented:
Yes. I went through all of the send and receive connectors just now and replicated the settings from a known good server at a different company. This fixed it. I'm not sure which setting was causing the problem.

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Ok, glad you got it fixed.  Keep your eye on the event viewer logs if one of the services is crashing.  Cheers!
SysToolsData Expert - Recovery,Backup,MigrationCommented:
Follow the manual steps to fix it using   

Hope solution works for you and gets the stuck emails.
AmitIT ArchitectCommented:
I doubt, the steps you have taken fixed the issue. Normally, error indicated means, your Exchange server having some issue. Did you restarted the Exchange Services? If yes, that might fixed it.
AaronSSHIT ConsultantAuthor Commented:
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