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NDRs - Status: 5.5.0 & Status: 5.4.0

I'm getting quite a few NDRs from my Exchange 2003 server. Some come back "Status: 5.5.0" and some come back with "Status: 5.4.0"

I've done some research into these errors which seem to be caused by the recipients mail server. If it was only 1 or 2 then I'd be happy to put the problem down to the recipient's server but the frequency makes me think there's something else going on.

I have a suspicion what it might be but could do with some advice. 10 days ago the domain/mail server got black listed (due to a virus). That was swiftly dealt with and we've been clean since then, but do certain mail servers and spam filters remember that and keep our domain name listed as being a high chance of spam?

If our emails were getting rejected because the recipients mail server thought it was spam, is a Status: 5.4.0 / 5.5.0 the sort of NDR you'd expect to see?

Finally, if my suspicious is right - is there anything I can do improve our domain name credibility?

Thanks.
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fred2k3
Asked:
fred2k3
3 Solutions
 
svijay_kCommented:
A Reverse DNS entry (FQDN) must exist for the delivering server and hostname should resolve to the
 same IP address. The PTR record helps to identify the source of emails and authenticity of the source server.

also ensure your server is not acting as open relay. Block incoming smtp connections at perimeter level itself.
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it_saigeDeveloperCommented:
Besides the Reverse DNS entry, you also want to ensure that your EHLO response returns the FQDN for the external domain.

Make sure that you have an spf record in dns and for an extra added measure, you could also implement DKIM signing (although DKIM signing is probably going to require a 3rd party piece of software).

You also want to ensure that you are not an open relay.

I run monthly checks on my mailserver at MXToolbox to ensure that we are not on any black lists.

-saige-
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gheistCommented:
If you have discrepancies between reverse, forward and HELO names about 10% of mails will sit in queue with temporary rejection and about 1% will bounce back right away.
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