Postfix start error Port 25: Address already in use

I experienced the same issue on Ubuntu 16.04.01 LTS. The following log message occurred in /var/log/mail.log and outbound mail was failing to be sent out:

postfix/master[5072]: fatal: bind 127.0.0.1 port 25: Address already in use

I received the following output when trying to kill sendmail:

$ sudo killall sendmail
sendmail: no process found

The following commands worked for me (no need to reboot the server):

sudo killall sendmail-mta
sudo service postfix restart

I had the same issue and what I did is only stop send mail, restart postfix and start again sendmail with the following commands:

service sendmail stop
service postfix restart
service sendmail start

Everything came back just fine.

Well the answer is profoundly unsatisfying but after burning some hours working on this today, I restarted the server and postfix is now running. Thanks all who lent some insight.