Last night I attempt to send emails from my ubuntu server to an address. I used the php's mail function with sendmail MTA and I managed to send some emails. At the beginning I send one email and then it wasn't working. I examined the configurations, if the sendmail MTA is running, port-forwarding...but all these was right. What I did was to uninstall the sendmail and all packages comes with it and then reinstall again. I tried to send and it was working properly. For testing I sent 3 emails and I received them. Today I open my server, try to send and nothing is delivered in my email address. Does somebody have an idea of what is the main problem of this?
Restart sendmail.
service sendmail restart
Then try to send the email again. Did this work for you?