Hi,

I was having the same issues configured sendmail and postfix both one by one however I faced no such issue in past on ubuntu14 tried many workarounds but at last ended using sparkpost to send mails via server applications.

Hope you can get a workaround with following this tutorial:

https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-on-ubuntu-16-04

Vivek Prism