Error sending email to alias with Postfix

I'm on Ubuntu 11.04 64bit.

I'm trying to set up Postfix on my VPS, which has been configured but when I send an email to an alias e.g. webmaster@domain.com it will send it to myemail@hotmail.co.uk. Now when I sent the email from my GMail account, I got this returned:

Delivery to the following recipient failed permanently:

    webmaster@domain.com

Technical details of permanent failure:
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: 550 550 #5.1.0 Address rejected webmaster@domain.com (state 14).

----- Original message -----

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
       d=gmail.com; s=20120113;
       h=mime-version:date:message-id:subject:from:to:content-type;
       bh=R1WtjVRWywfkWCR2g4QKbSjAfUaU9DAAMKbg9UAWqvs=;
       b=FiSfdhEaV4pEq/76ENlH4tvOgm35Ow3ulRg06kDYrIQTaDf3eOEgfSEgH25PjZuAj/
        7Hg1CL++o6Rt/tl80ZiR2AWekhA0zIn2JkqE7KssMG7WbBmMmbf8V9KDo2jOw+mZv+C/
        KDKsQ65AudBZ/NYLDDpTT7MkKf8DzqeGCKj9MAct6sHDoC0wCciXYxNfTf+MKxrZvRHQ
        oICTkH5LOugKW9wEjPF2AoO8X0qgYmTLYeSUtXxu46VeNKRBGmdRkkpPOoJlQN9ank7i
        SW6kU6M9bk2bYOgKwV/YPsaantmYlu1XdmYx+kWeJkNJAyYOfXfZZ8WUJhbbFFD9bZCi
        m/hw==
MIME-Version: 1.0
Received: by 10.101.3.5 with SMTP id f5mr783908ani.86.1334247306547; Thu, 12
 Apr 2012 09:15:06 -0700 (PDT)
Received: by 10.236.73.136 with HTTP; Thu, 12 Apr 2012 09:15:06 -0700 (PDT)
Date: Thu, 12 Apr 2012 17:15:06 +0100
Message-ID: <CAN+9S2aB=xjiDxVZx3qYZoBMFD4XuadUyR_3OYWaxw1ecrZmOQ@mail.gmail.com>
Subject: Test Email
From: My Name <myemail@gmail.com>
To: webmaster@domain.com
Content-Type: multipart/alternative; boundary=001636c597eabfd21504bd7da8fd

Now that I don't understand why it isn't working, my aliases are set up correctly – I see no error messages being produced in /var/log/mail.log or any other mail logs, which makes it harder for me to debug.

This is my postfix configuration (postconf -n):

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
config_directory = /etc/postfix
inet_interfaces = all
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
mydestination = $mydomain, $myhostname, localhost, localhost.localdomain, localhost
mydomain = domain.com
myhostname = localhost
mynetworks = 192.168.1.0/24 127.0.0.0/8
readme_directory = no
recipient_delimiter = +
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes

Does anyone know how to solve this specific issue?

EDIT:

I found this in my mail log (/var/log/mail.log):

Apr 13 21:56:03 giTzor postfix/smtp[22260]: 12E761796006: to=<myemail@hotmail.co.uk>, orig_to=<developer@mydomain.com>, relay=mx2.hotmail.com[65.55.37.120]:25, delay=1.5, delays=0/0.01/0.64/0.8, dsn=2.0.0, status=sent (250  <4f875bf3cd801@mydomain.com> Queued mail for delivery)
Apr 13 21:56:03 giTzor postfix/qmgr[1773]: 12E761796006: removed

EDIT:

This is the MX records at GoDaddy:

EDIT:

I've edited my A records and MX records:

I can send emails to my aliases with Google Mail, but I can't seem to receive emails from other sites such as Facebook, Yahoo, etc – even Hotmail accounts.