I can't access via VM webmail - in the past that's where I go if I have any outlook problems and I've always had the temporary measure of using it via there - I'm just getting the 'mailbox is not available at the moment'
Ok, the most likely reason is VM have locked the account for security reasons.
Try changing the password for the account as that may unlock it.
Sign in to My Virgin Media, go to My Profile and/or Manage Accounts depending on whether the account concerned is your primary account or an additional one. Select the account concerned and then click on Change to the right of password if a primary account or View/Change then change to the right of password for secondary accounts.
If this doesn't work then the VM Forum Team will be able to help you when they reach this thread in a day or so.
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