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If everything on your todo list will be implemented, then this can be the best contact form plugin 
I'd have 1 more request. If user won't set an e-mail address to receive messages from contact form, then it should use GS admin's mail address by default.
Thus changing admin's address won't force the user to change settings in contact form plugin.
There may be a small problem in future, when GS will allow multiple user management.
Then in contact form plugin default mail could be chosen from a drop down list, taken from all registered users.
edit: 1 more thing. Although there's 1 captcha, you could develop a simple mathematic captcha.
Something like generating 2 numbers, where 1 of them would be shown as verbal, and user would have to enter result of the arithmetic action (as a number or verbal).
Like: 2 * twenty one = 41 (or forty one)
Users would be able to choose between 2 captchas.
I'd have 1 more request. If user won't set an e-mail address to receive messages from contact form, then it should use GS admin's mail address by default.
Thus changing admin's address won't force the user to change settings in contact form plugin.
There may be a small problem in future, when GS will allow multiple user management.
Then in contact form plugin default mail could be chosen from a drop down list, taken from all registered users.
edit: 1 more thing. Although there's 1 captcha, you could develop a simple mathematic captcha.
Something like generating 2 numbers, where 1 of them would be shown as verbal, and user would have to enter result of the arithmetic action (as a number or verbal).
Like: 2 * twenty one = 41 (or forty one)
Users would be able to choose between 2 captchas.
Addons: blue business theme, Online Visitors, Notepad
