If you are talking about an imap account, it is possible the server spam filters have put it into the server 'Junk/Spam' folder. In which case you would need to logon to your webmail account via a browser and see if you can instruct the Spam filter to allow that particular sender.

Do you have any Message Filters that move emails to 'Spam/Junk' folder? If yes, then check to see if there is a common link. Do you have the 'Filter Log' enabled ? If yes, then in Message Filters, click on 'Filter Log' as it may offer some information on what filter set junk status as Junk or set up a move to junk. If no, then in Message Filters, click on 'Filter Log' and select the checkbox 'Enable the Filter Log' as it may be useful in the future if it occurs again.