Let's have look at the pathways taken by an email message you send as it leaves your national Internet infrastructure, and where it is exposed to surveillance.
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Demonstration:
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Note: Open wireless networks (those that do not require a password to connect) are just as vulnerable to surveillance by anyone connected to the same wireless router. They present an even greater risk since anyone within range can connect. Wireless networks protected by a password present a much greater challenge, since everyone's connection to the router is encrypted with an individual session key. You should be using a long WPA2 standard password (as opposed to a WEP key) and if your router doesn't have that option – time to upgrade!