1) Please check to see if they are getting any error messages back.  If it says "451 Blocked", please go to this article:
http://kb.maknet.com/index.php/article/cant-receive-e-mail-451-blocked

2) If they didn't get an error-message back, you need to check your spam folders (and whitelist your client):
http://kb.maknet.com/index.php/article/check-your-spam-folder-in-webmail