SMTP issue

I run Exchange 2003 on our server and it's been running well for months. About an hour ago we stopped having the ability to send external emails to most domains. I can send an email to gmail but not to hotmail. No software or server upgrades were preformed on our server.

The error replay we're getting is..

There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            < #5.5.0 smtp;550 OU-001 (SNT0-MC4-F46) Unfortunately, messages from weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to>

I checked the blacklist sites and we were listed on one which I had them delist us. I've also called our service provider (Verizon FIOS) and I'm being told it has nothing to do with them.

Any guidance would be greatly appreciated.
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John EastonConnect With a Mentor DirectorCommented:
It does look like your domain has been added to one or more Blacklists.  This is possibly on the recipients server, or a public block list.  It can also take time for changes to blacklists to update on the recipients server if they cache blacklist results for a period of time.
jfletchsterConnect With a Mentor IT ManagerCommented:
Each black list provider will have a diff refresh time, plus the Blacklists are cached on the recipent servers from some time to help with thier look-up times.
Most blacklists use DNS features to properage, so expect to take a minimum of 24-48 hours to update. Plus the additional time that each blacklist service might take to check before delisting.

Also you might want to make sure that you have resolved the problem for which you have been blacklisted, i.e. spyware on a pc, someone mass mailing or a spam bot on a pc.
becasue if you get listed again it will take even longer to delist and sometime needs a telephone call or a email to the blacklist company to get delisted.
Use your firewall to lock down traffic out-bound for port 25 to prevent any computer being able to send email out directly with out leaving from the exchange server.

Deny Source IP 'internal subnet', destination 'any', source port 'any, destination port '25' (with exception IP of exchange server)
dak11Author Commented:
Looks like it might have been a blacklist. I've also updated my firewall to block all traffic except my email server that could be sending emails.

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