Install SMTP on your Ubuntu server so that it can send out emails:
sudo apt-get install postfix
To configure your SMTP:
sudo dpkg-reconfigure postfix
Here are the most common options:
General type o mail configuration: Internet Site
System mail name: (your server name)
Root and postmaster mail recipient: Leave it emtpty to save these mails in /var/mail/nobody
Other destinations to accept mail for: You will probably get a comma separated list. Accept them.
Force synchronous updates on mal queue? No
Local networks: You will probably get a comma separated list. Accept them.
Mailbox size limit: Leav it as 0
Local address extension character: +
Internet protocol to use: ipv4