Outlook Express not sending to Yahoo & Hotmail addresses

[barrym]

Windows XP Professional, 512MB RAM, Outlook Express 6.0.
For the last few weeks, my OE can't seem to send my emails to any users with yahoo.co.uk or hotmail.com addresses.  ALL such messages come back with delivery errors from the Mail Delivery Subsystem.  Start of error message reads:  The following addresses had permanent fatal errors.  Message also sometimes contains: Deferred Warning: message still undelivered after 4 hours  This has only developed recently, and I have not knowingly added any new software or made major changes for some time.
PLEASE can someone help with this?  It is important that I am able to send to yahoo & hotmail users asap.  Thanks.  Barry

[Deerpark]

Outlook is able to send emails to non yahoo or hotmail addresses right? If that is the case I don't think the problem is with Outlook Express. It is more likely to be either a problem with your email provider or with Yahoo and Hotmail. I suggest you contact your email provider and ask them if they're having problems.

[barrym]

Thank you for responding to my query, and for being the only one!  Outlook Express DOES send out all my emails to anyone else except yahoo or hotmail OK - sorry I should have included that info in my original request. 
I don't think it can be my ISP as my other email accounts, on yahoo and hotmail, both work perfectly from the yahoo one.  I think it might be to do with my domain which I use as my email address in Outlook Express and I am taking it up with them as a first option.
Thank you for pointing me in the right direction.

Barry