Having the Mailbox Unavailable error on one of my email accounts. Creating this as requested in main post ( https://community.virginmedia.com/t5/Email/Webmail-403-Mailbox-Unavailable-errors/td-p/4294661 )
The email account is listed within the main VM billing account, password has been reset, which made no difference.
Thank you for your post. I'm sorry to hear about the issue you're having.
I will private message you so we can look into this.