How to fix exim Could not complete sender verify

Check if you can reach a remote server

dig a

If this timeouts then try over one of dns resolver directly

[~]# dig @ a

; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.2 <<>> @ a
; (1 server found)
;; global options: +cmd
;; connection timed out; no servers could be reached

Timeout means DNS resolver is dead

[~]# cat /etc/resolv.conf
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Popular DNS resolvers

Google DNS
Private and unfiltered. Most popular option.

Level 3 DNS

Old player that blocks malicious domains and offers the option to block adult content.
CloudFlare DNS
Private and unfiltered. New player.

Quad9 DNS
Private and security aware.
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