Had a similar problem. This helped me –
main.cf
myhostname = smtp.gmail.com
# Use Gmail SMTP
relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_mechanism_filter = plain
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_use_tls = yes
smtp_tls_security_level = encrypt
tls_random_source = dev:/dev/urandom
smtp_sasl_tls_security_options = noanonymous
and
sasl_passwd
[smtp.gmail.com]:587 [email protected]:pass
1
solved OS X Server with SMTP Relay