The mail module lets your users
with appropriate permissions send mail to registered site users. Emails
are also saved to the site database for future reference.

Mails
sent from the admin account are tagged with the site name and come from
the site admin email address. Mail sent from other accounts have that
account's email as a return address and are tagged with a name in the
form "username at site" where username is the user name of the user
doing the posting and site is the name of the site.

In administer » configuration » modules » mail
you can set up an introductory text for mail authors and a floor on the
number of words which may be included in a mail. This is designed to
help discourage the submission of trivially short mail messages.

User access permissions for mail

send emails to users:
Allows a role to post email to users. They cannot edit or delete
stories, even if they are the authors. You must enable this permission
to in order for a role to create a mail message.