Are you compliant with all these limits?

https://www.xfinity.com/support/articles/limitations-on-sending-email

Yes. Mail Merge sends them as individual e-mails, so the 1000 limit applies. It has been working for years until TB v115.

At link it says 1000 per day, but it may mean per 24 hour. So if you sent out 900 yesterday at 10AM and then tried to do the same today at 9AM, that's within the 'day' time frame, so only 100 would be allowed to send until after 10AM today.

Not relevant. These batches are in the 300 range. Also, it stops after 11 e-mails, then allows me to resume another 11 after I close TB, wait for the cache to clear, restart, and manually start the Send.

This is comcast /xfinity which is blocking you . The link below is useful - worth bookmarking - because it list as a load of common error server messages and yours is mentioned near the bottom. 452 - [Too many emails sent on this session] Comcast allows 1000 emails per session. This error results when that limit is exceeded.

At the bottom there is also a 'report an issue' button to click on if you need assistance. I would use it to find out what's going on. https://spa.xfinity.com/postmaster?faq=comcast-mail-errors

Thank you for the link. I looked for something similar, but couldn't find it. I'll try the assistance form.

However, I was nowhere near the 1000 message limit - only ~300 per day for 2 days.

Maybe I'm too focused on the words, but that error message says the message is too big, not that you are sending too many.

Per my original message, 5 - 9 kb each, text only, no attachments.