Gmail used to have a limit of 500 recipients per email message (or 100 if sent from an email client program).
It appears that maybe they just changed that limit from 500 to about 10. They still claim it is 500, but I don't think so!
I send weekly emails to a list of about 50 addresses. All was fine, until this week. This week's message bounced saying that the message was "blocked" to every single address, including my own. I received 50+ bounces, one per address.
After a few days of retrying, and looking for an explanation, I finally resorted to sending separate emails to everyone on the list, and they all went through, no problem. Golly.
But I got lazy near the end of the list, and my last message was addressed to 11 recipients -- and that one bounced. When I split it into two messages with half as many recipients each, they both went through. Therefore, I conclude that someone at Google has throttled the threshold down from 500 to about 10.
When the limit is exceeded, the bounce message is cryptic, saying little more than the fact that the message has been "blocked", and refers to https://support.google.com/mail/answer/69585 . All the bounces look the same and came from googlemail.com (I'm in the USA), even for the non-Gmail recipients. I think the messages never left Google and were rejected internally by Google.
followed by a severely truncated version of my email message, and no further diagnostic information.
In case it matters, messages were sent from Gmail's web interface (client software not involved), on Chrome.
What's going on? Has there been an announcement about such a change? Does it sound like a bug?