Possibly, it is trying to start sendmail before your network is up and running.
But, are you actually using sendmail? If you aren't using it, then disable it.
Code:
systemctl disable sendmail.service
But if you do use it, make sure the network is up before starting sendmail
You may need to start the NetworkManager-wait-online.service.
Code:
systemctl enable NetworkManager-wait-online.service