How can I revert my etc/postfix/main.cf file? I've already lost my etc/postfix/main.cf file in Yosemite. Could someone help me to revert or give me the link to download it?
If you've manipulated your main.cf already, check your backups to recover it.

Otherwise simply copy the default file: main.cf.default:

sudo copy /etc/postfix/main.cf.default /etc/postfix/main.cf


and replace the last two lines with:

readme_directory = /usr/share/doc/postfix
inet_protocols = all
message_size_limit = 10485760
mailbox_size_limit = 0
biff = no
mynetworks = 127.0.0.0/8, [::1]/128
smtpd_client_restrictions = permit_mynetworks permit_sasl_authenticated permit
recipient_delimiter = +
tls_random_source = dev:/dev/urandom
smtpd_tls_ciphers = medium
inet_interfaces = loopback-only