Exchange is blocking incoming AOL Messages (421 4.3.2 System not accepting network messages)

Hi there,

We're having trouble receiving a client's email messages that are being sent from AOL Mail to our Exchange server. AOL Postmaster replies with a rejection email listing our mail server and the following error: "421 4.3.2 System not accepting network messages (in reply to end of DATA command)" We've white-listed our address through our firewall but the message still can't come through. We're able to send outbound messages to the AOL account but not inbound. Error message below:

From: Mail Delivery System <>
Sent: Wed, Oct 16, 2013 2:40 pm
Subject: Undelivered Mail Returned to Sender


Your e-mail is being returned to you because there was a problem with its
delivery. The reason your mail is being returned to you is listed in the
section labeled: "----- The delivery status notification errors -----".

The line beginning with "Diagnostic-Code:" describes the specific reason
your e-mail could not be delivered.  The following lines contains the
RFC822 header of the original email message.

Please direct further questions regarding this message to your e-mail

--AOL Postmaster

----- The delivery status notification errors -----

<>: host[XXX.XXX.XXX.XXX] said: 421 4.3.2
    System not accepting network messages (in reply to end of DATA command)

Anyone run into this or know the problem?

Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Simon Butler (Sembee)ConsultantCommented:
The NDR is not an Exchange NDR.
Do you have something between Exchange and the internet that is scanning the SMTP traffic? If so then that is the cause of the problems. Ideally firewalls etc shouldn't be scanning SMTP traffic, they rarely make a good job of it.


Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
Zoldy2000Author Commented:
Thanks, that thought led us to the problem.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today

From novice to tech pro — start learning today.