SendGrid sudden deliverability drop on a shared IP

Symptom: rising deferrals, more spam-folder placement, or blocks, with unchanged content and lists.

Cause candidates:

1. Shared-IP neighbor damage. Confirm: check whether the sending IP's reputation signals dropped while your own engagement stayed flat.
2. Your own engagement decayed (stale list, frequency jump). Confirm: opens down and spamreports up on your campaigns.
3. Authentication broke (DNS record changed, domain expired). Confirm: re-validate domain auth.

Fix:

1. Rule out 2 and 3 first; they are cheaper than moving IPs.
2. If the IP is the problem and you send real volume, move to a dedicated IP. The doc recommends dedicated IPs for high volume and for control over who shares your reputation.
3. Warm the new IP gradually; do not cut all traffic over on day one. An IP idle or new gets evaluated by ISPs on volume and engagement.
4. Keep authentication, list hygiene, and engagement discipline on the new IP, or you will recreate the problem alone.

Verification: deferral rate falls and inbox placement recovers over the warmup period. If nothing changes, the cause was never the IP.

The reverse DNS doc: on a shared IP, reputation is the aggregate of all traffic on it; a dedicated IP is recommended for high volume or control over IP neighbors. The warmup doc: ISPs evaluate new IPs by volume and engagement, and an IP idle 30+ days needs re-warming. Sources: https://docs.sendgrid.com/ui/account-and-settings/how-to-set-up-reverse-dns | https://docs.sendgrid.com/ui/sending-email/warming-up-an-ip-address