I have virgin router past 5 years or so and have recently been unable to send email from any of 4 btinternet.com email accounts when attached to home network. Other email such as iCloud all work fine. I can send email when attached to other wifi or 4G networks. Does BT block outgoing IP addresses?
Just to add this is when using Outlook or Applemail using mail.btinternet.com with authentication port 465. I can send using webmail