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Inbound email to SBS2008 Exchange not being received

Shannon Mollenhauer
Last Modified: 2012-05-09
Client's Exchange mail internally between users works fine. External senders trying to send mail to users are not getting through. System is on a Comcast cable internet connection with dynamic IP, so we use DYNDNS services. Ran MXToolbox SMTP check. Here are the results:

 Not an open relay.
 0 seconds - Good on Connection time
 10.265 seconds - Not good! on Transaction time
 OK - resolves to
 Warning - Reverse DNS does not match SMTP Banner

Session Transcript:
HELO please-read-policy.mxtoolbox.com
250 remote.designerconcepts.biz Hello [] [62 ms]
MAIL FROM: <supertool@mxtoolbox.com>
452 4.3.1 Insufficient system resources [5070 ms]
RCPT TO: <test@example.com>
503 5.5.2 Need mail command [5070 ms]
221 2.0.0 Service closing transmission channel [62 ms]

System has worked flawlessly for about 5 months. This just happened over the last 24-36 hours. What do I do?
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