Mail related information that I could not put only into articles like postfix or bash.

user accounts email

Source

Some application (e.g. 'at') can send updates/status via mail to the user account's email address. This address is always 'user@\<host>' and therefore delivered locally.

A user can specify a forward email address where the emails are being forwarded to.

username$ cat ~/.forward
\username,inbox@example.com

Alternatively an email alias can be specified in /etc/mail/aliases like this:

username: inbox@example.com