Recently,I discovered that emails from one person were not arriving in the Inbox, but they were on the server of my ISP. Outlook was apparently blocking this person's emails, whether sent from their office address, their home gmail account or from their mobile phone. No other emails have been blocked from any sender.

The emails were not going to Junk Mail. A check of Tools>Junk Email Protection>Blocked senders showed that none of the sender's addresses were listed; The sender's domains were not listed in Safe Domains either.

More than an hour's investigation by the ISP's support team could not find any reason why emails from this one address received in the Webmail box of the ISP should not be automatically downloaded by Outlook with other mail.  The sender had the office IT expert check for any spurious artifacts on the emails which would have made Outlook reject them, but there was nothing.

Emails sent from my Outlook to that person - at office or home are delivered correctly, but replies get only as far as the ISP server. Outlook is functioning correctly otherwise, but this seems to have started from the time of Office 2011 security updates in 2014. Is there a secret flag to block these emails I am missing?