Postfix issue: Relay access denied and problem with sending mails

[Debianer]

Hi,
I've just installed postfix and popa3d - debian packages.

1. When I try to send an e-mail from Gmail account to me:

user@domain.com

I'm getting error:

Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 554 554 5.7.1 <user@domain.com>: Relay access denied (state 14).

2. When I try to send an e-mail from Postfix to Gmail it normally, go out, but I am not getting this e-mail on my Gmail account.

My main.cf is:

myorigin = /etc/mailname # domain.com

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no

append_dot_mydomain = no

readme_directory = no


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

myhostname = my computer static ISP hostname
mydomain = domain.com
mydestination = $myhostname
mynetworks = 192.168.0.0/24
inet_interfaces = all

alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases

disable_dns_lookups = yes

mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +

What should I do?