Blacklist & block list for emails/domian/IP

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I run an IT company, we are busy : We support Many companies and run a lot of projects each year. We support many exchange server.
We are a full control company: Sale of Servers to MSP supporting the companies.

I have noticed that all the companies who were on exchange 2003 etc (older platform) did not have any issue and never got black/block listed.
As we move along: We use SSL Certificates from Godaddy, 2013/2010 Exchange : 4443 Port forwarding to Exchange, Properly configured exchange and also Barracuda as a SPAM filtering service for all clients as well.
We go to the point that Port 25 is blocked on the firewall.
Black/Block lists are taking toll on our support.

Is there something we are missing?
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Eric CIT Director / Project Manager
Commented:
are you sure it has anything to do with the version of Exchange?

have you set up DKIM?
SPF records?
is DNS set up properly?
are you using a smarthost? What IP are the outgoing messages using? have you checked mxtoolbox.com (for example) and if the IP is on a blacklist, does it tell you why?
is rDNS properly set up?
Email is not the only source of why you would get black listed.

I've had ip's blacklisted because of malware backdoors using various ports.
Most Valuable Expert 2014
Commented:
If it was the version of Exchange, then it would have been resolved by now.
Furthermore if an Exchange server was compromised you tend to notice as it fills the queues.

As already pointed out, the most common reason for blacklisting is malware. Block port 25 outbound for everything but the Exchange server, or get a smart hosting service and have Exchange servers route through that, so the actual IP address isn't an issue.

You need to look at WHY you are being blacklisted - that will give you an idea as to the problem.
Most Valuable Expert 2014

Commented:
Question has been answered, with no follow up from asker.

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