Email not being delivered by Comcast and Google to some addresses

I have a location served by Comcast.  I have a cable modem with an Apple Airport Extreme router.  The IP address provided by Comcast is  Several computers are on the network.  Most use Mac OSX Lion.  But, we also have OSX Snow Leopard and Windows XP on Parallels.

About a month ago, all OSX Mail clients using and would send email to some addresses and the recipients never received the e-mail.  There are no error messages.  Email to Gmail, comcast, yahoo, etc. are delivered.  Email to users at companies that own their own domain like are not delivered.

Eudora on Windows XP worked perfectly all the time.

If I take a Mac laptop to another location served by Comcast, the email works perfectly.  I even brought a Mac laptop from another location to the "bad" location, and its email did not arrive at the recipient.  So, it is something specific to the IP address at the bad location:  We have had the IP for a long time even though it is a dynamic IP.

This is a normal home that does not send any spam.

What on earth could be going wrong?  Is it an smtp mail spam filter?

Now, here is some extra info.  I removed the router and connected one Mac directly to the cable modem.  This forced a new IP address.  Miraculously, email worked perfectly.  So, I swapped the Airport Extreme with another I had.  This forced a new IP.  Guess what, the email works perfectly.

So, the problem is tied directly to the IP address and I guess Mac email client.  Can anyone explain what might be going on.  Comcast was no help.

It has to be an IP block combined with the Mac email headers I think.
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nociSoftware EngineerCommented:
Have you checked the blacklist providers?

This is what comcast has registered for your address block. location.
ALL mail needs to be forwarded through the comcast server...

So you may need to check if you resolve mailsending yourself or not.
tfexAuthor Commented:
We always use authenticated smtp.  We also use SSL and use the standard ports.

I have Comcast at two other sites that we use to send mail for our own domain.  We have never had a problem there.  We have problems only at this one site and it is using both and

I took my laptop to the "bad" site and sent mail to all the e-mail addresses that failed using my  All worked properly.

So, I am not sure what is different about earthlink in this equation.  Or for that matter, Eudora on windows (success) and Mac Email (fail).

Thank you for the insight, but it does not solve the problem.  The bigger issue is if this reoccurs, I want to know how to get this fixed without resorting to changing routers or MAC Address cloning to play IP roulette.
nociSoftware EngineerCommented:
The IP address you mentioned is put on a so called blacklist. If the receiver of mail checks that blacklist, it might block the mail.
If it doesn;t use this blacklist it might let mail pass.
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tfexAuthor Commented:
How do I check the blacklists?
nociSoftware EngineerCommented:
There are many blacklist organisations, I gave one example before (spamhaus).
Others are DNSBL, SORBS, spamcop etc.
And they all have various lists that are about known or suspected sources of spam.
And some providers subscribe all their uses on dialup lines f.e. to such blacklists because they want users to use their central servers as smarthost. somewhat indexes on the best known databases so this URL:
gives an index of your address and where it is known, as does:

Here is a report about BL's or services that can tell when when you enter a blacklist

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tfexAuthor Commented:
Thank you very much.  Now I have an idea of where to start looking if this happens on the new IP address we got by changing the MAC address for the router.  I guess if it happens again, there is some virus or other malware at work.  We run current anti-virus and spyware filters so that should not be the case.
tfexAuthor Commented:
The new IP address has now been flagged as spam.  I simply cannot understand why.  There is nothing spam-like happening from this IP.  Anyway, I have configured a work around for me to receive on my earthlink account.  In the webmail interface, I set the spam preferences to whitelist the address.  I also noticed that the default action for spam is to delete it.  I have changed this to store it in the junk folder.  This will allow me to monitor the spam to see if other senders are having the same problem.

There does not seem to be a well defined method to get off of spam lists.  There is also no good description of why the IP is on the list in the first place.
nociSoftware EngineerCommented:
The problem is there is no well defined description of Spam.
The best one is: If the receiver doesn't want it it is spam.

If enough receivers flag it as spam even legitimate mail will become spam.
If too may receivers are with one ISP there is a larger risk of becoming a spam sender because too many mails arrive in too short a time frame, from the same source.

Also the real spammers do their utmost best to look like legitimate mail, at least from the perspective of automated counter-measures.
And in filtering spam is needs to be a balance between avoiding and accepting the existence of false positives [ legit mail assumed to be spam and spam assumed to be legitimate ].
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