How is Ubuntu able to send alerts to external e-mail addresses?
Solution 1:
This happens because Ubuntu desktop and server has by default (at least on 16.04 and onward) postfix.service enabled and running. You can confirm this by issuing the following command in terminal:
systemctl --type service --state running | grep postfix
The sending of mail works because postfix does a simple DNS lookup for the MX record for the domain the mail is destined for. The receiving server simply takes the mail it got sent and maybe does a few checks on it and puts it on the receivers inbox. So no SMTP configuration is needed to send an email.