OK. I live in Mexico and our email server is prodigy.net.mx
My wife has contacts in various parts of Mexico. Sometimes she gets
emails from some to these people and sometimes not. They get emails
from her.

We seem to get emails ok from the USA and other countries. I have
CCNOW as my credit card collecting agency. I do very few sales on the
Internet, but one I had talked to the customer never showed. I called
her and she had ordered after our last phone conversation. The Company
CCNOW sends me an email of the orders. That email did not come.

Could it be a hacker is intercepting our emails and they do not reach us?

I called our technical support at prodigy.net.mx and they told me to
go into their web site and send myself an email from my account via
the web site.

That one arrived. 

Oh, we use Outlook Express version 6.0
What to do?

Add their e-mail addresses in your "Safe List". This guarantees that
their e-mail will not be filtered out as JUNK.

Ask your e-mail provider on how to configure your "Safe List". (It
varies with each e-mail server)

Or an alternative would be to ask them to use a specific "Subject
Line" such as your name, name of the business etc. and enter this word
in your junk mail filter and configure it to automatically deliver
messages with this specific "Subject Line" to be delivered to your
Inbox automatically.