I see Google results indicating that error is to do with Google accounts on Android devices but you appear to have posted from a windows device. This is the desktop forum, so can you confirm the operating system and Thunderbird version please.

I am receiving error message: R88 authentication failed. I am on Windows 11. How do I correct my SMTP login credentials?

Turns out my ISP had changed the email security options about August 19, 2025. Apparently, I "missed the memo." Once I got in touch with the ISP's tech support, they were very helpful and gave me the new security options. My emails are again filling the either-net. My thanks to Matt for his response.