Exchange 2003 ent edition - whats up with my ISP or microsoft

Recently my ISP enforced us to change the whole networks external IPs.  I did that on thursday.  Obviously has to change the external exchange IP address too.  Since last night one of the domain user is having issue as under,  I wanted to send an email to the same domain as well but the same issue as under.  I can send emails to lots of domains.  Never had this issue before.  Where do I seek help?  is microsoft doing something or my ISP?

One of my domain user is trying to send an email to a specific domain and the message bounces back as:
 < #5.7.1 smtp;550 5.7.1 Service unavailable; Client host [] blocked using Blocklist 2, mail from IP banned; To request removal from this list please forward this message to>
amanzoorNetwork infrastructure AdminAsked:
Who is Participating?
Check your ISP has a valid reverse DNS record for your IP,
Check if this IP isn't blacklisted
Check your DNS public records and modify any SPF you might have created.
yo_beeDirector of Information TechnologyCommented:
Do you have a PTR for your public MX record?
Is this a static or dynamic ip-address being issued by your ISP?

If the receiving end's server has the reverse look feature enabled and you do not have a PTR then the message will be rejected.

I had a client that used Cable internet ISP that issued dynamic addresses for business class back in 2000 and AOL and other domains rejected the e-mails because they were part of a dynamic pool of addresses.  I had to use the SMARTHOST to forward all mail to my ISP's SMTP and relay off that.

These are some ideas to look at.
amanzoorNetwork infrastructure AdminAuthor Commented:
Thanks Guys:
Microsoft has released the IP, at least I am able to send to that particular domain.  I am unable to telnet to my excahnge as:
telnet 25
telnet externalIPofEXchange 25
could not open connection to the host on port 25
Is it good or bad?
Help plz

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If your public IP is now you default MX for the domain you're hosting, then it is not good: You should be able to telnet your external IP on TCP Port 25 (SMTP).
Telnet 25 will probably not work but telnet 25 should work (host can be SMTP, MAIL, MX or anyname you of the host you created in your DNS Zone.)

This is done by configuring your router/firewall to accept incoming connexion on public IP port 25 and NAT'ing the connexion to the Exchange server.
Depending on the firewall (ISA, Arkoon, fortigate, netasq, linux box,...) you must modify the "Public IP" object created to reflect the new public IP you got.
Turn on filtering logging and check errors to get a more verbose output and analysis.

amanzoorNetwork infrastructure AdminAuthor Commented:
My MX records are held by my ISP.  Do you think thats the reason, also when I nslookup,, it gives me the preferred mail server of my ISP who is taking care of my mx of my mail.  Is it good?
ahh sure,  it is good !
Your ISP holds your MX, analyzes mails and then you either go "pop the messages" with a pop conector or they forward these messages to your mail server using your ..... public IP! (In this case, only your ISP is allowed to telnet your exchange server from the outside.)

If this is the case, you must give them the new public IP to forward the mails.

Of course you still need to check your firewall rules and hosts unless your ISP also manages your firewall.
amanzoorNetwork infrastructure AdminAuthor Commented:
Thanks Guys.
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