smtpmail: Authentication
3 Authentication
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Most SMTP servers require clients to authenticate themselves before they
are allowed to send mail. Authentication usually involves supplying a
user name and password.
If you have not configured anything, then the first time you try to
send mail via a server, Emacs (version 24.1 and later) prompts you for
the user name and password to use, and then offers to save the
information. By default, Emacs stores authentication information in a
file ‘~/.authinfo’.
The basic format of the ‘~/.authinfo’ file is one line for each set
of credentials. Each line consists of pairs of variables and values. A
simple example would be:
machine mail.example.org port 25 login myuser password mypassword
This specifies that when using the SMTP server called ‘mail.example.org’
on port 25, Emacs should send the user name ‘myuser’ and the password
‘mypassword’. Either or both of the login and password fields may be
absent, in which case Emacs prompts for the information when you try to
send mail. (This replaces the old ‘smtpmail-auth-credentials’ variable
used prior to Emacs 24.1.)
When the SMTP library connects to a host on a certain port, it
searches the ‘~/.authinfo’ file for a matching entry. If an entry is
found, the authentication process is invoked and the credentials are
used. If the variable ‘smtpmail-smtp-user’ is set to a non-‘nil’ value,
then only entries for that user are considered. For more information on
the ‘~/.authinfo’ file, 
auth-source (auth)Top.
The process by which the SMTP library authenticates you to the server
is known as “Simple Authentication and Security Layer” (SASL). There are
various SASL mechanisms, and this library supports three of them:
CRAM-MD5, PLAIN, and LOGIN. It tries each of them, in that order, until
one succeeds. The first uses a form of encryption to obscure your
password, while the other two do not.
Info Catalog
smtpmail: Emacs Speaks SMTP
smtpmail: Top
smtpmail: Encryption