Update Just when long-suffering Virgin Media customers thought their spam woes had been fixed, it seems the firm's inbound mail server is now blocking the delivery of mail.
The grey-listing anti-spam measure has been affecting users for several weeks, with many complaining of delays in receiving emails.
One customer wrote to The Register to explain the problem thus: "The first time round users get a 421 error, then in theory when the sending server attempts redelivery the mail should be accepted.
"This is to help sort out the mail servers from the spam bots out there. Spam bots typically won't retry; mail servers always retry unless they get a definite 5xx error.
"Virgin's solution continues to reject the mail servers when they come back."
At the end of last year, Virgin Media overhauled its botched spam filtering system, which has resulted in UK businesses and customers missing thousands of legit emails. The issue had persisted for over a year.
Virgin apologised on the forum for the latest SNAFU: "Please accept my sincere apologies for this. We have had a few other reports of this particular issue and Postmaster, along with Liberty Global email engineers, are investigating."
The Register has asked Virgin for a comment. ®
Updated to add
A Virgin Media spokesperson has been in touch to say: "Virgin Media takes internet security extremely seriously in order to protect our customers from viruses and cybercrime. As a result, our email filtering system will block emails it believes are suspicious.
"In this case, we believe a small number of users experienced issues when trying to send mail to Virgin Media email addresses because they were categorised as 'spam'. We've now rectified this and apologise for any inconvenience caused. We continue to review the system to ensure it's working effectively, while keeping our customers safe online. If users experience similar issues in sending mail to Virgin Media email addresses, we advise them to visit netreport.virginmedia.com/netreport."