mail.protection.outlook.com rejecting our email

We host our own Exchange Server.  We only have trouble sending email to Outlook.com addresses.  Here is the error -

mail.protection.outlook.com[x.x.x.x] said: 451 4.7.500 Server busy. Please try again later from [x.x.x.x]. (AS77712200).

We are not on any block list and have submitted our ip address to Outlook.com Deliverability Support.  Also joined SNDS and JMRP per their recommendations.  

Any suggestions?
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Information TechnologyAsked:
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CESNetwork AdministratorCommented:
Does your sending server have correct PTR and SRV records in place?
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
This message 451 4.7.500 Server busy suggests the server you're connecting to really is busy.

If there was a problem with your outgoing DNS + SPF + DKIM, I'd expect a very different message.

Open a support ticket with Outlook + ask them how to resolve this. Likely this will provide you with the fastest resolution.
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viktor grantExchange ServersCommented:
HI,

Which Exchange version do you have?

It seems that there is some delay in sending. Do you use any proxy?

Is your environment hybrid?

Cheers
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Information TechnologyAuthor Commented:
We have Exchange 2013.  

Microsoft has NOT been much help.  Here is their response last week...

Hello,
 
We have completed our investigation of your sending IP (x.x.x.x) and determined that there are no active blocks against it.  
 
However, we found that some messages were being filtered based on the recommendations of the SmartScreen® Filter which is resulting in them being delivered to user's junk mail folder rather than their inbox.
 
We have confirmed that your IP is eligible for temporary mitigation based on our mail policies.
 
Please note: The mitigation process may take 24 - 48 hours to replicate completely throughout our system.
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Information TechnologyAuthor Commented:
We were being throttled by Outlook.com addresses.  Exchange was trying to send the health check emails outside.  Fixed that and now everything goes through fine.
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
Author reached resolution on their own.
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