These instructions were written for Sendmail 8, it might work with different major and minor version numbers but consult the Sendmetric documentation for your version to verify. Make sure to backup the existing files referenced below before you modify them in case you need to revert your changes.
1. Modify /etc/mail/sendmail.mc
Make sure the following lines exist in the file:
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl
2. Modify or Create /etc/mail/access
Add the line below to the top of this file, replace username and password with the SMTP username and password that you created. For more information on SMTP users see the Getting Started Guide:
AuthInfo:smtp.sendmetric.com "U:username" "P:password" "M:PLAIN"
3. Run the following commands from the CLI (make sure to backup your existing sendmail.cf file referenced below first):
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
/usr/sbin/makemap hash /etc/mail/access.db < /etc/mail/access
4. Restart Sendmail