Symptom: deliverability issues or rejections concentrated on a dedicated IP, especially with strict receivers.
Cause: reverse DNS was never set up, or the A record changed without updating rDNS.
Confirmation:
1. Look up the IP's PTR record. If it does not resolve to your domain, rDNS is missing.
2. Check the A record for the sending domain points at the SendGrid dedicated IP. Both directions must agree; the doc calls this the circular check.
Fix:
1. You need a dedicated IP; rDNS cannot be set up on shared IPs.
2. In the console, set up reverse DNS for the IP and add the DNS record SendGrid provides, plus the A record mapping your domain to the IP.
3. Validate in the console and wait for DNS propagation.
Verification: PTR lookup on the IP returns your domain, the A record returns the IP, and the console shows the rDNS record validated.