Having reached a provider before by e-mail now all my messages are being returned
by the system say as non deliverable but will keep trying.  I get the same message  
about every 4 hours so can a provider turn her e-mail or some how block me?

I have had to block a crazy wife from messaging me and IMing me --- she thought her hubby was seeing me so she was trying to snoop. It's possible you were blocked, consider what the content of prior messages was and think if any part might have caused alarm. In the mean time, move on to someone with a working email and phone (like me!).

There are free applications that verify the validity of email addresses.

Looking and reset the settings of your email account...Or you must have accidentally reported her emails to you as a SPAM once...or either she must have put u on her SPAM folder...Or, how about   contacting her directly and let her know  as she as well might not be aware of what was going on in her emails...

The email header will tell you why it's being returned. It may be an error code but it will translate to something like mailbox full, account deleted, server down, routing loops ...