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 - 126.96.36.199 resolves to
Warning - Reverse DNS does not match SMTP Banner
250 remote.designerconcepts.biz Hello [188.8.131.52] [62 ms]
MAIL FROM: <firstname.lastname@example.org>
452 4.3.1 Insufficient system resources [5070 ms]
RCPT TO: <email@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?