Are you sure they sent you an e-mail? Are they? How do they know it
actually got *sent* from their mail server (and not just get an OK
status back to their e-mail client)?
Using any filters or rules in your Gmail account or in Outlook?
Checked if they are in your blocked senders list in Outlook?
Using any anti-spam software?
Did you ever check the Inbox folder on the mail server by using Gmail's
web interface (and without Outlook running) to check if it ever does get
those e-mails?
Checked the junk and trash folders up on the mail server by using
Gmail's web interface to your account?