Sharepoint Shared Folder Verification Code Blocked - Not Getting Emails At All

Hi, I've read numerous threads that talk about adding various email addresses to the Safe Sender list, etc to resolve the problem where you share a Sharepoint folder with a link and then the recipient cannot receive the verification code.

But the problem we have is that the verification codes seem to be getting blocked by our email provider altogether before we even see them.  We are experiencing this, and so are the clients for whom we use this method for file sharing.

They all suggest checking the Spam folder and adding specific emails to the Safe Sender list. 

We've done all of that and now have a nice collection of "no-reply@" emails in our contacts. Since we've never successfully received one, we don't know what email address to add so we've just been going off support forum posts.

I've also tried (temporarily) disabling all spam filters and other automatic email processing.

I can watch what happens with my email provider's web client open on one screen while I'm clicking the link to get a verification code - a few seconds later I can see that something happens in the web client but the email never appears - not in the Inbox, not in Spam and not anywhere else. This is why I think it's hitting something either within Microsoft or within our email provider that's blocking the message.