Microsoft has blocked Brit hosting outfit TSO Host's bulk email domain, meaning anyone trying to send large quantities of mail over its infrastructure cannot deliver it to an Outlook or Hotmail address.
Irate resellers got in touch with The Reg to complain that their own customers were moaning that emails sent to any Microsoft domain were bouncing.
Those resellers were using TSO Host's infrastructure for their operations – including the hosting firm's dedicated bulk IP.
"Rather a lot of our clients have mailboxes with TSO/Paragon," reflected one. "Apparently TSO's 'senior team' have requested the removal a while ago but Microsoft aren't interested."
TSO couldn't be bothered to reply when The Register messaged them asking what was going on, despite them inviting us to do so. Microsoft is probably still rewriting Bing in the hope of seeing relevant search results that tell it what TSO Host is, but we wait with bated breath for their reply.
Messages sent over TSO Host's infrastructure to an Outlook, Hotmail or Live email address bounce. The bounce message is pretty unequivocal:
550 5.7.1 Unfortunately, messages from [18.104.22.168] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [VE1EUR02FT020.eop-EUR02.prod.protection.outlook.com]
TSO Host's website contains a status update from nine days ago at the time of writing, which says: "We have requested a delist from SORBS for the Daily mail IP range 22.214.171.124/24 and they have confirmed that the IP is not listed any more." [sic]
Combined with the IP address in the bounce message above, it's not looking good for TSO Host and its ability to deliver mail to Microsoft-controlled addresses.
In its status page message, TSO Host added: "We are still awaiting an update on the matter with Microsoft. Once we have information from them, we will further update."
Email issues are nothing new, though they tend to consist of ISPs overblocking potential spam messages – as infamously kept happening with Virgin Media over the past few years.
A fed-up source caustically commented: "So, as long as people with TSO mailboxes don't have any friends on Hotmail you're fine. D'oh!" ®