[jks]

My experience is that Gmail accepted my emails fine... until one day it didn't. Then some time later it worked again.

I registered for their Postmaster Tools, which says

    No data to display at this time. Please come back later.
    Postmaster Tools requires that your domain satisfies certain conditions before
    data is visible for this chart. 
    Refer to the help page for more details.

The help page has no useful information. I suspect that I sent too little mail for it to register in their systems at all.

Outlook was even worse, and I just told my Outlook users to change providers.

Eventually I capitulated and got Google Workspace, and now everything gets delivered perfectly.