I have just started having the above issue, can receive emails but not send them and get the followingmessage when trying to.
An unknown error has occurred.
Subject 'test' Server Error: 421 Server Response: 421 csmtp1.tb.ukmail.iss.as9143.net cmsmtp Sorry your IIP-deleted. is active in our local abuse blacklist. Contact Virgin Media abusedesk for delisting requests. Server: 'smtp.blueyonder.co.uk' Windows Live Mail Error ID: 0x800CCC67
Would appreciate this resolved as soon as possible.