Mail Stopped Both Ways — “451 Temporary Local Problem” and the One DNS Resolver Nobody Checked
A cPanel server stopped sending and accepting mail at the same time. Roundcube showed “SMTP Error (451) Failed to add recipient – Temporary local problem” and Track Delivery was full of failed sender verifies. The cause was one line in resolv.conf pointing at a DNS server the network could not reach. Here is the proof, the fix and the reboot trap.