GoDaddy is rejecting mail with URLs that appear in the Spamhaus PBL. As this thread on the Amazon EC2 forum notes, this is creating false positives, causing nonspam mail to be rejected. Here’s what GoDaddy reportedly said about this policy:
Unfortunately, our system is set to reject mails sent from or including links listed in the SBL, PBL or XBL. Because the IP address associated to [REMOVED] is listed in the PBL, any emails containing a link to this site will be rejected. This includes plain-text emails including this information.
If this is true, it’s utterly broken.
Spamhaus explicitly warn that this is not to be done, on the PBL page:
Do not use PBL in filters that do any ‘deep parsing’ of Received headers, or for other than checking IP addresses that hand off to your mailservers.
And more explicitly in the Spamhaus PBL FAQ:
PBL should not be used for URI-based blocking! Consider the false positive potential: legitimate webservers hosted with services such as dyndns.com or ath.cx! Or consider that ISPs and other networks are encouraged to list any IP ranges which should not send mail, and that could include web servers! Use SBL or XBL (or sbl-xbl.spamhaus.org) for URI blocking as described in our Effective Spam Filtering section. Use PBL only for SMTP (mail).
Critically, the PBL now lists all Amazon EC2 space, since Spamhaus interpret Amazon’s policy as forbidding email to be delivered via direct SMTP from there. (Note — email, not HTTP.)
With this filter in place at GoDaddy, that now means that if you mail a URL of any page on any site hosted at EC2 to a user of GoDaddy, your mail won’t get through.
Note: this is much worse than blocks of SMTP traffic from EC2. In that case, an EC2 user can relay their legit SMTP traffic via an off-EC2 host. In this case, there is no similar option in HTTP that isn’t insufferably kludgy. :(