Throttling
ISPs slowing down acceptance of mail from a sender — often the first deliverability warning before harder filtering.
Throttling is when a receiving mail server intentionally slows down acceptance of email from a sender — accepting messages more slowly than the sender wants to deliver, or returning temporary failures (4xx codes) that force the sender to retry over a longer window. Throttling is often the first signal of a deliverability problem, before mail starts landing in spam or being rejected outright.
ISPs throttle when a sender's reputation is in question or when the sending volume looks suspicious (sudden ramp, unusual patterns). Gmail's reputation system in particular uses throttling as a graduated response — bad senders get progressively slower acceptance before harder filtering kicks in.
Throttling is usually invisible at the campaign level — most ESPs absorb the retry overhead and just report normal delivery rates. Spotting throttling early requires looking at per-MBP delivery latency and queue depths. If sends to Gmail are taking 4 hours when sends to other MBPs take minutes, you're being throttled and there's likely a reputation issue brewing.