How to restore the Terminal mail program to default settings
There must be two files main.cf.default and master.cf.default which you can use to overwrite the messed up ones, thus resetting them to the distribution default values.

cp /etc/postfix/main.cf.default /etc/postfix/main.cf
cp /etc/postfix/master.cf.default /etc/postfix/master.cf


If you can't find the default ones, you may try to copy them from another computer.

Also, you may type postconf -d which will print main.cf default parameter settings instead of actual settings.  Specify -df to fold long lines for human readability (Postfix 2.9 and later). Read more here.