If your domain mail is hosted on mail server in a different domain, you should tell your sendmail about it. Otherwise if you try to send a mail to a user in your local domain, you will get an error from sendmail at /vat/log/mail.log:
It means that sendmail is trying to find user, that is mentioned in a message, on your local server and can’t find that user, because we use mail server that is hosted on different IP-address. To solve this problem we should tell sendmail to search for mail users with our domain name on the remote server.
Open /etc/mail/sendmail.mc for edit:
sudo nano /etc/mail/sendmail.mc
Add this two lines to the end of the file:
define(`MAIL_HUB', `your.domain.com.')dnl define(`LOCAL_RELAY', `your.domain.com.')dnl
Press “Ctrl + X” when you are done and answer “Y” for question about saving a file.
Now we should reload sendmail service:
service sendmail reload
And try to send mail again. If you have done everything right, emails should go through remote mail server without errors.