What is sendmail referring to here?

It's referring to this page from the readme, which tells you how to specify your hostname. It's warning you that your hostname won't work outside your local network; sendmail attaches your hostname as the sender of the message, but it's going to be useless on the other end because people outside your local network can't find the machine ixtmixilix. You should specify a hostname that can be resolved from anywhere, like ixtmixilix.example.com

For me on Ubuntu 14.04 that solved the problem:

$ sudo nano /etc/hostname

Change your host name to something like my-domain.XXX

Where XXX = .com OR .net OR .some

Then sudo sendmailconfig updated all the rest You can test your configuration so:

$ sendmail -v [email protected] < test.mail

I used my private domain email box as services such as gmail block these kind of email: 050 550-5.7.1 [87.68.229.9 12] Our system has detected that this message is 050 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 050 550-5.7.1 this message has been blocked. Please visit 050 550 5.7.1 https://support.google.com/mail/answer/188131 for more information. u3si25544809wje.160 - gsmtp

Where test.mail is a file containing whatever message you wish to send

On Debian 8 (Jessie) works:

$ sudo hostnamectl set-hostname yourdomain.com

Then test it this way:

$ hostname yourdomain.com