Email delivery delayed

We're having email delays to some recipients when sending from Outlook via our Exchange 2010 server. Our email system uses an external spam filter run by Microsoft at  filter.itsafemail.com. Here is the error message we are receiving:

Delivery is delayed to these recipients or groups:

steves@somedomain.com (steves@somedomain.com)

Subject: ABC Contribution 2/16-2/22

This message hasn't been delivered yet. Delivery will continue to be attempted.

The server will keep trying to deliver this message for the next 1 days, 19 hours and 51 minutes. You'll be notified if the message can't be delivered by that time.
PerfishentAsked:
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PerfishentConnect With a Mentor Author Commented:
This problem turned out to be a natting issue. Our MSP had natted our email server's internal address to two different external addresses. Some email was being routed through the correct address and some was going through the wrong address. Correcting the natting problem fixed the issue.
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pcelbaCommented:
It is obvious behavior when the e-mail recipient (or its mail server) does not accept incomming mails due to various reasons/errors, e.g. user mailbox is full, the maintenance is in progress, or even the antispam filter refused the incomming message.

If the error is recoverable then the sender attempts to deliver the message several more times as you may see from the message.
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PerfishentAuthor Commented:
This problem started occurring suddenly and with multiple different domains that our company sends to. Something must have changed somewhere that is causing our email to be delayed and ultimately not delivered. This looks like we've been blacklisted or graylisted somewhere, but I can't find our domain listed on any blacklists. Our email domain is mail.aplusbenefits.com, although everything goes through the Microsoft filter filter.itsafemail.com.

I'm also wondering if there could be a problem with the reverse DNS lookup either for our domain or for the filter.itsafemail.com domain.
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pcelbaCommented:
Ask some of your recipients to send the incomming mail header. It should contain the trace and spam score which will tell more.

You may also temporarily disable filter.itsafemail.com
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tmoore1962Commented:
Yes verify your mx records, and HELO response settings on the Exchange servers also would want set up spf record in dns if you haven't already.  You can go here to test your connectivity.  Is your exchange server set up to send all outgoing emails to the spam filter server or does it send directly to recipient?   If sending directly its probably a reverse look up problem.  
https://testconnectivity.microsoft.com/
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PerfishentAuthor Commented:
We discovered the solution on our own, and none of the solutions submitted by others were correct.
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