check the /var/log/maillog , there should be entry such as below
xxxxxx sendmail: o9T3l3Pe030683: to=
Then we try to nslookup the mail server, by dig xxx.xxxxx.com. It should show that the connection is time out.
; <<>> DiG 9.3.4-P1 <<>> xxx.xxxx.com
;; global options: printcmd
;; connection timed out; no servers could be reached
Next, we can resolved this by setting up name server or put the mail server into the /etc/mail/mailertable .
First by checking the /etc/nsswitch.conf to see if we are using hosts file.
#hosts: files dns
Then do the following to create the entry in the mailertable.
then we hash the file to the mailertable.db
makemap hash /etc/mail/mailertable < /etc/mail/mailertable
after that, restart the sendmail by running service sendmail restart