I really don't like having sendmail running on a webserver, but some features of just don't work if it can't send email (user registration, for example). Still, offers support to send email through external servers instead if a local mailer.
/wp-includes/pluggable.around line 377, change
/wp-includes/class-mailer.around line 155, set your host:
You may also need to set a username and, and tell WP to attempt authentication. You'll see those in the lines below the hostname variable.
Easy (but took 4 go's to get right!)