Exchange 2016 Cannot Send Mail

Fresh build of Exchange 2016 CU10 on Server 2016. Forgetting outbound for the minute, mailboxes on this server cannot send messages to each other on the same database. They can't even send messages to themselves. They just go to the Drafts folder.

Here's what I've tried:

• In the send connector, used MX records and routing via smart host. Neither makes a difference.
• Tried using external DNS to resolve, no differnce
• Deleted the send connector altogether (in theory shouldn't need it for this), no difference
• Ensured all name resolution is correct (DC configured in Exchange and on NIC, PTR records resolved, logs show resolution is ok)

Not really sure where else to go from here. I've scoured every log, I keep seeing this recurring error:

421 4.3.2 Service not available

This shows up in multiple logs, but I can't see any errors or reasons why this might be the case. I also can't figure out what service is not available, in fact I can see SMTP handshake with the smarthost in the protocol logs. Running Test-Mailflow is equally unhelpful:

RunspaceId         : 3ae5c152-51a0-46f8-b886-c6e6d80a033b
TestMailflowResult : *FAILURE*
MessageLatencyTime : 00:00:00
IsRemoteTest       : False
Identity           :
IsValid            : True
ObjectState        : New

Ok, FAILURE, but why? Adding -Debug -Verbose doesn't make any difference either, it just takes longer and displays the same information.

Get-ServerComponentState shows everything as 'Active' except:

ForwardSyncDaemon
ProvisioningRps

Not even sure I need 2016, at this point I'm wondering if I admit defeat and install 2013 instead. Anything else I've missed?

EDIT:

I found this blog post which (among countless others) seems to describe a similar situation. I have followed through the steps in here and subsequently messages are no longer stuck in the drafts folder. Now they are actually going to sent item,s but they are not yet appearing in the recipients inbox.