It was a real PITA to get sendmail to cooperate with sending via a secure SMPT relay. I found instructions on several websites that resulted in a 95% solution.
First you will need to install sendmail-cf. It is a separate package the is require to modify recompile the sendmail config.
dnl# This is the default sendmail .mc file for Slackware. To generate include(`/usr/share/sendmail-cf/m4/cf.m4')dnl VERSIONID(`Linux Home Automation FC6 Gmail')dnl OSTYPE(`linux')dnl define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun')dnl define(`confTO_IDENT', `0')dnl FEATURE(`use_cw_file')dnl FEATURE(`use_ct_file')dnl FEATURE(`mailertable',`hash -o /etc/mail/mailertable.db')dnl FEATURE(`virtusertable',`hash -o /etc/mail/virtusertable.db')dnl FEATURE(`access_db', `hash -T /etc/mail/access')dnl FEATURE(`blacklist_recipients')dnl FEATURE(`local_procmail',`',`procmail -t -Y -a $h -d $u')dnl dnl FEATURE(`always_add_domain')dnl FEATURE(`redirect')dnl EXPOSED_USER(`root')dnl FEATURE(`authinfo',`hash /etc/mail/authinfo')dnl LOCAL_DOMAIN(`yourlocaldomainhere.com')dnl define(`SMART_HOST',`smtp.gmail.com')dnl define(`RELAY_MAILER_ARGS', `TCP $h 587') define(`ESMTP_MAILER_ARGS', `TCP $h 587') dnl# define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl dnl# define(`confCACERT_PATH', `/etc/pki/tls/certs') define(`confCACERT', `/etc/pki/tls/certs/ca-bundle.crt') define(`confSERVER_CERT', `/etc/pki/tls/certs/sendmail.pem') define(`confSERVER_KEY', `/etc/pki/tls/certs/sendmail.pem') define(`confCLIENT_CERT', `/etc/pki/tls/certs/sendmail.pem') define(`confCLIENT_KEY', `/etc/pki/tls/certs/sendmail.pem') dnl # dnl # masquerade not just the headers, but the envelope as well dnl # FEATURE(masquerade_envelope)dnl dnl # dnl # masquerade not just @mydomainalias.com, but @*.mydomainalias.com as well dnl # dnl FEATURE(masquerade_entire_domain)dnl dnl # MASQUERADE_AS(`yourdomain.com') dnl FEATURE(genericstable, `hash -o /etc/mail/genericstable') dnl dnl GENERICS_DOMAIN_FILE(`/etc/mail/genericsdomain')dnl dnl # dnl # Leave these at the end, sendmail prefers these last (for the most part) dnl # MAILER(local)dnl MAILER(smtp)dnl MAILER(procmail)dnl
Rebuild the csendmail.cf with m4 sendmail.mc > sendmail.cf
You will need to create /etc/mail/authinfo.
AuthInfo:smtp.gmail.com "U:smmsp" "I:username" "P:mysecret" "M:PLAIN" AuthInfo:smtp.gmail.com:587 "U:smmsp" "I:username" "P:mysecret" "M:PLAIN"
This is built into authinfo.db with makemap hash /etc/mail/authinfo < /etc/mail/authinfo
Be sure to chmod o-rx /etc/mail/authinfo.
You will also want to edit /etc/mail/local-host-names to include your hostname.
At this point you should be able to mail outside the system using sendmail and the gmail SMTP servers.