Why am I not getting email from one particular sender? .

Have a look in

Mail > Preferences > Rules

and see if his email address is listed in any of the rules. If so, just delete the rule, or change the action associated with it.

Go to Mail Preferences, and click the "Rules" icon. Make sure there are no rules that move these messages to another folder (you can just uncheck the rules to deactivate them).

Also try searching for the sender's email address to see if it turns up - be sure to click "All" in the mailbox row after typing his address in the search box.

And depending on what email service you use, you may want to check to make sure there are no "rules" or "filters" in the web site - for instance if you use gmail, you can set up filters in gmail that "archive" a message before Apple mail ever sees it - the mail is still there, you just wouldn't see it in your inbox. The only way to fix it is to log in through the gmail web site.

Another possibility is that you accidentally turned on "threaded" mail view, which is extremely confusing if you don't expect it. Instead of grouping by date, the messages will be grouped by subject line - and if the person sending this message is replying to an old message of yours, it will continually group those new messages together with the old ones. Click the "View" menu, and check to see if the "Organize by conversations" option is turned on or off. Or maybe it's even simpler - maybe your mail is no longer sorted by date - what does it say at the top of the list of messages?

Of course there's alwys the possibility that he has the wrong email address in his contacts, so keeps sending to the wrong address over and over again. He should try manually typing your full email address, and not accepting the "autofilled" suggestion.