Error : Mail Server Upgrade in Progress. Message Queued

When sending mails from server to domains hosted in the same server, we get the following error message in exim_mainlig
Mail Server Upgrade in Progress.  Message Queued.

Solution is to edit exim.conf, comment the following lines in /etc/exim.conf to fix the issue
#temp_defer:
#  driver = redirect
#  allow_defer
#  data = :defer: Mail Server Upgrade in Progress.  Message Queued.
#  verify = false

and restart exim.