Magento – PayPal IPN not triggering Order Status updates

I'm setting up my first Magento system, using 1.8.0.0, running on a HostGator Windows server.

I've configured IPN on our PayPal account. However, the payments never get communicated to Magento, and all orders are stuck at "Pending Payment".

Currently, System->Configuration->Payment Methods ->PayPal Payments Standard is associated with the Business Sandbox email, has Payment Action set to "Sale", Sandbox Mode = "Yes", and Debug Mode = "Yes".

On PayPal's side, the IPN History shows each transaction (I've tried in both Sandbox and Production) as "Retrying". There is no HTTP Response Code listed for any of them.

The Notification URL in PayPal shows http://www.mydomain.com:/shop/Index.php/paypal/ipn/, which appears correct to me.

After turning on error logs and debug mode for the payment method, I get a log file for my sandbox attempt entitled payment_paypal_standard.log.

The contents of the file reflect the actual sandbox order:

2014-01-02T18:19:13+00:00 DEBUG (7): Array

(

[request] => Array
    (
        [business] => ****
        [invoice] => 100000006
        [currency_code] => USD
        [paymentaction] => sale
        [return] => https://www.mydomain.com/shop/Index.php/paypal/standard/success/
        [cancel_return] => https://www.mydomain.com/shop/Index.php/paypal/standard/cancel/
        [notify_url] => http://www.mydomain.com:/shop/Index.php/paypal/ipn/
        [bn] => Varien_Cart_WPS_US
        [item_name] => My Shopping Cart
        [lc] => en_US
        [charset] => utf-8
        [amount] => 7.99
        [tax] => 0.00
        [shipping] => 5.00
        [discount_amount] => 0.00
        [item_number_1] => Bacon4NFHS
        [item_name_1] => Bacon - Nitrate Free Hickory Smoked 4 oz.
        [quantity_1] => 1
        [amount_1] => 2.99
        [item_number_2] => Flat Rate - Fixed
        [item_name_2] => Shipping
        [quantity_2] => 1
        [amount_2] => 5.00
        [cmd] => _cart
        [upload] => 1
        [tax_cart] => 0.00
        [discount_amount_cart] => 0.00
        [city] => recipient_city
        [country] => US
        [email] => sandbox_customer_email@test.com
        [first_name] => customer_fname
        [last_name] => customer_lname
        [zip] => customer_zip
        [state] => customer_state
        [address1] => customer_address1
        [address2] => 
        [address_override] => 1
    )

[__pid] => 5440
)

I'm new to both PayPal IPN and Magento, so I may be making an obvious mistake, but I've also been having a ton of problems with the Magento install that were caused by problems with the auto-installer package HostGator provided.

Why are my payments not getting recorded by Magento?