Go here.
http://www.dnsreport.com
Enter the person's email domain name in the field on the left and click "dns
report ". When the next page comes up...just remember... Red is bad. If
there is alot of red, then it could be something to do with the DNS of that
person's site.
Click the back button...Now... put YOUR domain name in and check out your
own DNS. If there is alot of red on your dns results... it could be your DNS
problem.
You could also put the same info in the "mail test" field and see what
results you get there as well.
I run my own email servers and this is one of the places that I use to
determine the cause of the problem you described, if one of my clients
reports a problem to me. If there is something on that page that you don't
understand.... post if back here and I'll try to help.