If your ISP blocks your outgoing 25 port or you want to use google mail as smpt relay host here
is a short tutorial:
Make sure that you have sendmail installed.
Add those lines in /etc/mail/sendmail.mc:
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo')dnl
Create file /etc/mail/authinfo and add this line:
where user is your gmail user and pass is your google email password.
After that chmod 0600 /etc/mail/authinfo then:
#makemap hash /etc/mail/authinfo < /etc/mail/authinfo
#m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
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