I have two email addresses on my VM account. I use one and my wife, the other. Recently my wife's account can't send emails using Outlook. we get the error message 550 5.1.8 sender domain invalid vm420.
I've tried logging into her account directly on the VM website and she can send from there. It only seems to be via Outlook that we have the problem.
I've checked all her Outlook settings and other than the address, all are the same as mine.
Thank you so much. After reading your comments I went back to the account settings and checked again - somehow, @virginmedia had been misspelled! I have no idea how it got changed, but it works again now.