Sending and receiving in webmail is not exactly the same as using IMAP, POP and SMTP servers. Your messages sent with TB might be running afoul of the spam filters applied by some recipients. Things that might trigger a spam filter include signatures with remote images, messages with many links, embedded remote images etc. Recipients might not even be aware of such filtering, but I'd first ask if your mail is being directed to a Spam/Junk/Bulk folder. Also, try sending a simple plain text message with no signature and see if that gets through. It's also possible your IP is being blocked as an anti-spam measure, due to no fault of yours.

The emails that never arrived were just plain texts - no attachments or graphics or add-ons or links. As I said, messages to my son, e.g., are successful. I'll check with my stock broker to see if his emails are in a Spam folder. Thanks for your interest.

do you have a signature? That is a prime issue with spam filters, especially if it mentions social media

I don't have a signature. I'm 84 and retired and live in a senior living apartment. I use a desktop computer with Windows 11. No cellphone.usage except to control my hearing aids.

The condition happens when I'm contacting the apartment management or my stockbroker. In either case there may be attachments of data tables I generate using LibreOffice, but typically just a straight text.

Thanks for your interest.