550 5.7.1 Unfortunately, messages from [18.104.22.168] weren't sent
This is caused by "bad neighbour effect" - another customer of our mail host who happens (by chance) to be on the same IP address as our mail server has sent messages which Microsoft deem to be spam. Rest assured that UK VoIP Forums never sends spam emails and that we never pass on any information (such as email addresses) to any third parties! We are working to resolve this issue and in the meantime any activation emails that are returned by Hotmail.com or Outlook.com will result in the account associated to those email address being manually activated by the site admin.
Thank you for your patience and understanding.