Encrypted mail is rejected by mailserver

Question

I started sending mail encrypted with openPGP (Thunderbird + Enigmail) this month and it has worked perfectly for several weeks. Today I attempted to send an encrypted message, but received this rejection from "Mail Delivery System" instead:

[Return Code 553] sid: PVV41n00f1kmamQ01 :: 5.3.0 nlpi180 DNSBL:RBL 521< 173.201.193.233 >_is_blocked.__For_information_see_http://att.net/blocks

The 173.201.193.233 is a GoDaddy IP, and DNSBL:RBL 521 means that the message was identified as spam (by who I do not know). I am using GoDaddy as the registrar for the domain name that I send the email from.

Since this first rejection, all attempts to send encrypted email have been rejected with a similar rejection message from "Mail Delivery System", but unencrypted messages go through fine.

Does this mean GoDaddy/AT&T/someone else is monitoring my emails, or did I cause this by activating an adaptive spam filter somewhere? I can see how encrypted emails would look like spam (or garbage, rather) to an unknowing spam filter. If it is my fault in unknowingly activating a spam filter, is there a good way to allow encrypted emails without completely disabling the spam filter?

Answer 1

Well, I think this is GoDaddy spam policy implemented. There are quite a few people with same problem:

In 2003, the Can't Spam Act was implemented. Since then, large email hosting companies like GoDaddy have had to constantly balance the filters on their servers. If a certain email account gets flagged as spam, sometimes the entire server gets shut down until GoDaddy can either adjust the filters or weed out the problem customer who is spamming. This causes a lot of other emailers whose emails go through that same server to have an interruption in their service. Therefore, Godaddy (and other email hosting companies) are constantly walking a fine line. If they make the email filters too strict, the server overreacts and treats our harmless emails as spam or a virus. If they make the filters too loose, then the whole server may be shut down because of a spammer. When I asked why it goes through if I try to resend the email, he said it's because different servers have different filters and he reminded me that it is a constant balance they will always be working on....
(check below links for more)

here: http://social.technet.microsoft.com/Forums/office/en-US/136986d0-4ff3-44aa-a545-af3a775386fe/outlook-error-552-message-rejected-for-spam-or-virus-content-no-virus-with-and-without?forum=outlook
and here: https://forums.digitalpoint.com/threads/go-daddy-hosting-email-error-issue.2371308/

some people changed Godaddy to another hoster to fix this problem.