Tor doc says :
You can also get bridges by sending mail to firstname.lastname@example.org with the line "get bridges" by itself in the body of the mail. You'll need to send this request from a Gmail, Riseup!, or Yahoo! account, though — we only accept these providers because otherwise we make it too easy for an attacker to make a lot of email addresses and learn about all the bridges."
Can we explain why a Gmail or Yahoo account is more secure ? Gmail is under Patriot Act and has been releasing info to government agencies.