Login to the server SSH as root. Make a sender whitelist file.
Take a backup of the exim conf and open it.
cp -p /etc/exim.conf /etc/exim.conf.BKP
Add the line on top of the file
addresslist whitelist_senders = wildlsearch;/etc/exim_whitelist_senders
Now search for the line
require verify = sender/callout
Comment that line and add the following below that.
!verify = sender/callout=30s,defer_ok,maxwait=60s
!senders = +whitelist_senders
Save conf file.
Add email address to the /etc/exim_whitelist_senders file one by one. Wildcard is also acceptable here, Eg: *@domain.com