I use iCloud very casually for email, mostly relying on Fastmail for personal/business mail and IDG’s Outlook-connected system for Mac 911 messages. I’d never come across this, but sure enough, Apple has a support document that notes you may send no more than 200 outgoing messages a day with no more than 100 recipients per message. The daily cap is 1,000 recipients.
I knew there was a limit, but not like this.
Source: In case you didn’t know: iCloud email has limits on daily messages and recipients | Macworld