Seems to be a problem with some hosting providers the last day or so ...
Make sure that you have the current May 31, 2015 USPS shipping module:
https://www.zen-cart.com/downloads.php?do=file&id=1292
Next, what version Zen Cart are you using?
It sounds like there is a communications issue with your hosting provider and being able to send out/in on port 80, best to contact them ...
You can also try this file and unzip it and add to your site in
/extras
then run it as:
your_domain.com/extras/curltester.php
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