Email > Email Configuration > Protocol Configuration > Protocol Settings(SMTP) > Denial of service protection section. There is an option for "maximum number of recipients before a failure response is given." Change it from no limit to whatever you would like for the max recipients. There are a couple of things to look out for on this. If you do not want to do this globally, use protocol presets. The failure response given is a 400 level error, so a legitimate sending server will attempt to deliver to the rest of the recipients based on its retry schedule.
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