More email changes have been applied today, using a radically different method.
Microsoft decided to follow Google into requiring OAuth2 authentication for all email interaction, which is very problematic as PHP does not natively support this technology and can only make it work with a lot of additional work, and Simple Machines Forum doesn't support it without its own load of extra work and new code. Micronsoft's deadline for this is coming up Saturday and I've been scrambling to find how I can keep the forum running with email support. Fortunately, I discovered an OAuth2 proxy that can run as a service on my server and translate on-the-fly any basic authentication emails generated by the forum into OAuth2 secured emails.
This is working in my tests, and also has allowed me to switch the forum email system back to sognadigitalmuseum@gmail.com, from outlook.com. However, its new and may present problems.
If you notice any emails not being received, let me know.