"451 4.7.500 server busy" from mail.protection.outlook.com

Hello.
I have a ticket on Microsoft about my Symantec SMG (anti-spam) is receiving the  "451 4.7.500 server busy" from mail.protection.outlook.com (Hybrid exchange 2010 / Office365 -and Postfix for some days) when I try to send a email to @hotmail.com (my send connector send to SMG. It has to be this way, and it was working until two days ago).

This question is the same point:
https://www.experts-exchange.com/questions/29071178/mail-protection-outlook-com-rejecting-our-email.html

The Accepted Solution is:
"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."

what does "Exchange was trying to send the health check emails outside.  Fixed that and now everything goes through fine" mean?
LVL 1
adrecalAsked:
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Valentina PerezExchange ServersCommented:
HI Lacerda,

I do not understand "Exchange was trying to send the health check emails outside.  Fixed that and now everything goes through fine" . Exchange and all email server always verify if the email is sent outside, it checks if the email is received...

I hope the author replies to explain his solution...

Regards
Valentina
adrecalAuthor Commented:
Not solved yet :-(
Same problem, only when try to send email to @hotmail domain
Hemil AquinoNetwork Security EngineerCommented:
"451 4.7.500 server busy"


This mean few things, either you're sending too much email or Microsoft is throttling your session and thus gets busy, that is called congestion.

Now, few question what you changed your public IP address lately? Here is what you can do - contact Microsoft to have a temporary exclusion for your Office 365 IP throttling based on your IP.

I do throttle connection to some of my customers to avoid bottle necks.
Hemil AquinoNetwork Security EngineerCommented:
go to services and see if you have active directory domain and services running.
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