Apologies in advance for posting an amateur question in this forum, and also for taking so much space to express it.

I use a local hosting company for my family's and business's e-mail accounts and manage the server more or less on my own.  The inbound service is POP using SSL for authentication.

I book a lot of Air NZ tickets via their Web site and have always received the ticket receipts promptly at my account me@mydomain.co.nz.  Last week I booked a whole raft of flights and didn't receive a single receipt.  At more or less the same time I requested e-mail confirmation of the seat assignments on three of these flights, and those confirmations came through promptly to me@mydomain.co.nz. 

All of the bookings show up on "My Bookings" on the Air NZ Web site as well as on the Air NZ app on my Android phone.

Yesterday I went to the Web site and ordered, manually, one-at-a-time, the missing receipts to be sent to me@mydomain.co.nz.  Nothing came through.  When I say nothing came through - nothing showed up in my Thunderbird client, nor in my account on the server, nor in any spam or junk folder on the server or in my client. 

Today while I was on the phone with a lass at Air NZ she sent one of the receipts to an alternative address, mycompany@mydomain.co.nz.  It came through almost straightaway.  She tried resending it to me@mydomain.co.nz.  Nothing came through.  Ergo, the problem is mine, not Air NZ's.

But there are several things I don't understand here.

Why has this behaviour happened all of a sudden?

Why has it happened even though I have no local filter on me@mydomain.co.nz (other than an old filter screening out mail from an Indonesian spammer)?

Why aren't other senders' messages disappearing in the same fashion?

How do I investigate it?

The cPanel version is 56.0.36 and I use Horde in the cPanel environment.

Thank you for any assistance you can offer.