Problem emailing verizon users

I have used Windows Mail under Vista successfully for several years.  Now, in
the last 2-3 months, people with verizon.net email addresses no longer
receive my emails.  Everyone else does. I can receieve their email, however.
Can anyone help with this?

What happens when you reply to a received message? Does it never get there?

What happens when you reply to a received message?  Does it never get there?
Do you get a rejection message?  Did the recipient check the server side
spam filters?

steve