Emails are not delivering and Exim log /var/log/exim_mainlog reports "linear search: Permission denied"
This typically occurs if someone changes the group to root on files in /etc/ that need to be read by Exim. These files with 640 permissions need to be set to mail as the group or email will not deliver.
[root@cptestvm ~]# grep ^hostlist\ trustedmailhosts /etc/exim.conf
hostlist trustedmailhosts = lsearch;/etc/trustedmailhosts
These owner/group will cause emali to fail.
[root@cptestvm ~]# ls -al /etc/trustedmailhosts
-rw-r----- 1 root root 0 Feb 18 07:42 /etc/trustedmailhosts
with an error like
2021-02-24 05:12:48 failed to open /etc/trustedmailhosts for linear search: Permission denied (euid=47 egid=12)
This can also happen if someone adds a custom domainlist or hostlist without the proper permissions, user and group as well.
The fix is simple. The output provides the user and group id needed with 644 permissions. The group is enough to allow Exim to access the file though. Group ID 12 is usually mail so this is what you would do to correct the group.
chgrp mail /etc/trustedmailhosts
Nothing needs to be restarted afterward.