Increase number of email recipients

I don't know the specifics of your ISP or other servers you are connected to but from Google's web site about gmail:

"Recipients per message sent via SMTP (by POP or IMAP users) or the Gmail API

Addresses in the To, Cc, and Bcc fields of a single email. Includes email sent using smtp-relay.gmail.com or smtp.gmail.com*

Limit 100"

https://support.google.com/a/answer/166852?hl=en

But it could be a limitation by your ISP or Network Admin.

Try going down to 99 recipients.

Googling (really DuckDuckGo) shows that ISPs limit number. Usually number in one email and sometimes they also limit number during 24 hour period to make it harder to SPAM.

Maybe your ISP is limiting the number of recipients as an anti span measure. What is their limit?