Without seeing logs my best guess is the is the system doing a reverse DNS search on the connecting IP address. With Umbrella blocking the lookup your logs probably only show the IP address and would probably be why you can't search for the domain.
Looking up your example on google, you can search your logs for the below IP.
> mail.ekacitra.com.
Server: dns.google
Address: 8.8.8.8
Non-authoritative answer:
Name: mail.ekacitra.com
Address: 103.229.73.118