Exchange 2010 limit for incoming messages with many recipients ?

Out customer have their own Exchange server 2010 in their office. Incoming mail are first sent to an external server for spam filtering.
That server is a Linux machine running postfix.
After spam checks and similar is done, the email are sent to the exchange server which has a receive connector configured to accept incoming emails only from the external server.

This works fine, but there are one problem. Often when email are sent to the customer with more than one recipients, for example one TO-address and one CC-address, the exchange server refuse one of two recipients.
Later on, when the postfix server retries the refused address, it works. But the customer complains about delayed mails...

Log file from postfix:
I Aug 29 08:22:59 vmi65459 postfix/smtp[5190]: B398DD80AFB: to=<prename.surename@domain.com>, relay=none, delay=532, delays=531/0.03/0/0, dsn=4.4.1, status=deferred (connect to mail.domain.com [XXX.XXX.XXX.XXX]:25: Connection refused)

Is there a limit for number of concurrent connections for incoming mail which I can increase at the Exchange server?
Martin_RadboAsked:
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JAGDIS SCEOCommented:
You have to change the receive connection rate limits in exchange server side.
Try this with the help of this document
https://technet.microsoft.com/en-us/library/bb125140(v=exchg.140).aspx
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Martin_RadboAuthor Commented:
Thanks for your answer.
It turned out to be a limit at the hosting provider for the external spam server. They had set a limit of only 1 smtp-connection at the same time for port 25 :-(
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