The problem may be caused by the following reasons:
Spam filter settings If you set Exclusive or High filter level, most of your email may end up in your Spam folder. Please check if you have the "Delete immediately" option in the Found Spam section enabled. If you enable this option, all emails that are detected as spam are deleted. However, if your filter level is too high, even regular messages may end up in Spam and get deleted.
Wrong email address The sender of the email might have misspelled your email address, please have the sender make sure that your email address is typed correctly.
You're using Challenge/Response Spam Protection If you use Challenge/Response Spam Protection, emails from people who are not in your contact list may end up in the "Unverified" folder before the sender completes the verification process. Please check the "Unverified" folder, select the message and click on the "Approve Sender" button, if the sender is your friend. Next time he/she sends you a message, it will go directly to your Crawler.
Server error There is the possibility that either the sender's or your mailserver may crash. In this case email may not be delivered. Please ask the sender to resend the email.