How can I revert my etc/postfix/main.cf file?
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