Problems setting up Postfix to relay to Gmail

I am running Ubuntu 8.10 and trying to set up Postfix to relay all email to my Gmail account. I can't seem to be able to get the relay to work properly for some reason and I would really appreciate it if someone could point me in the right direction.

When I send an email to my email address, I get an “Undelivered Mail Returned to Sender” error email with the following error: “relay access denied (in reply to RCPT TO command)” or “Relay access denied (state 14).”

In mail.log I see errors like this one:

NOQUEUE: reject: RCPT from mail.domain.com[###.###.###.###]: 554 5.7.1 : Relay access denied; from= <username@domain.com>to= <username=domain.com>proto=ESMTP helo=<mail.domain.com></mail.domain.com></username=domain.com></username@domain.com>

Here is my main.cf file:

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname = li53-52.members.linode.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = li53-52.members.linode.com, localhost.members.linode.com, , localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all

# Gmail
relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_use_tls = yes

Here is my sasl_passwd:

[smtp.gmail.com]:587 username@gmail.com:password

Any suggestions?