My guess: Sending email to a lot of recipients when you're not a large email provider can end up with you getting throttled / needing to apply throttling to ind

My guess: Sending email to a lot of recipients when you're not a large email provider can end up with you getting throttled / needing to apply throttling to individual servers to prevent blocking. In the past when I still operated my own email servers I often saw list emails minutes before on e.g. gmail.