Login to the server SSH as root. Make a sender whitelist file.

touch /etc/exim_whitelist_senders

Take a backup of the exim conf and open it.

cp -p /etc/exim.conf /etc/exim.conf.BKP
vi /etc/exim.conf

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

Restart exim