Symptom:
After 2016 CU 3 install, Mails are stuck in queue
Mail stuck in queue LastError : 421 4.4.0 failed to parse smtp response from Mailbox Transport Delivery: string did not match smtp response suffix regex <250 2.0.0 OK[resubmit=False][Microsoft.Exchange.Transport.MailboxTransport.SmtpInClientHostname=xxxx
Cause:
Exchange 2016 server with CU3 are not able to do SMTP connection with the server running lower version of Exchange CU
Resolution:
Patch all Exchange 2016 server with CU 3.