554 Your access to this mail system has been rejected due to the sending MTA's poor reputation.

[BizGuy]

I got this message when I sent an email to a domain that happened to be hosted by GoDaddy.  The email was returned rejected with the error message above.  After a wild goose chase, the solution is:  if you have a Verizon dynamic IP address, make sure you use Verizon's outgoing mail server or you look like a spammer.

Here is the blow by blow and the fix:

GoDaddy screens out email against spam by checking one or more third-party blacklists.

     - You can get your IP address at whatismyip.com

     - You can check it for blacklists at senderbase.org

     - Sure enough, my IP address was showing up as POOR in a blacklist hosted by SORBS or spamhous or pbl.spamhaus.org

Since Verizon randomly assigns dynamic IP addresses, my first assumption was that a spammer had previously used my IP address and ruined its reputation.  So I turned my router off and on and tried to get a new IP address, but it just gave me the same one.

So I called Verizon tech support to ask them to give me a new IP address.  After some confusion (the moment they heard the word "blacklist" they just assumed that I was complaining about being on *Verizon's* internal blacklist of customers who send spam - a whole different issue that got us off track since they just tell you repeatedly to go to their whitelist request page) they understood.  They helped reset my IP.  I checked the new IP and once again it was blacklisted!  And a third time too!  So we gave up.

After the call, I researched more about SORBS.  It turns out that EVERY dynamic IP address range they detect goes right onto the SORBS DUHL list.  They reason is that they feel all mail from dynamic IPs is inherently suspicious because spammers can use pools of dynamic IP addresses to hide.  So they made a big list of dynamic IPs.

But here is the subtlety:  the way a host like GoDaddy uses this DUHL list to screen out spam is not to eliminate all email from dynamic Verizon IPs entirely.  Instead, they check if the mail is coming a mail server owned by Verizon.  If so they let it through.  If it comes from an unknown mail server then they block it. 

In short SORBS-DUHL is not REALLY a blacklist; it is a caution list.  Most people are fine because most people use the outgoing mail server that mnatches their dynamic IP provider and so GoDaddy will accept them.

As it happens, I have been using a non-Verizon mail server and that explains why GoDaddy's algorithm blocked me.  

So I simply changed my mail client to send out mail through the Verizon mail server (smtp.verizon.net, SSL on, port 465).  The problem was solved immediately and I could once again send mail to domains hosted by GoDaddy.