Re: unable to sent email on blueyonder.co.uk address.
Many thanks for confirming this. While I thought it might be the case, I prefer not to simply assume things.
Virgin have a number of different servers relating to e-mail. The outgoing ones in some cases aren't matching the incoming ones. While we've seen apparently legitimate cases of the VM304 error on the forums, we've also seen at least two other cases such as yours. It's not a password error.
SMTP error 525 is Authentication denied with VM304 giving the reason. If you are able to receive mails, then you definitely should not be getting this specific error.
I've added this thread to a list of threads we're monitoring behind the scenes (while Superusers don't work for VM we do work with them to highlight possible issues) and also flagged the thread itself for the Forum Teams attention.
Only use Helpful answer if your problems been solved.