>If I enter a valid email address that's just typo'd, the result is the same: the activation email bounces. //
No, you're excluding the set of emails that are entered incorrectly and thus are not valid. The result for those is not the same as if the UI included a simple test (such as your "ambitious" example).
1) Without UI validation:
- 1.1) Email address entered correctly -> activation email sent
- 1.2) Email address entered incorrectly but forms a valid address -> activation email sent to wrong address
- 1.3) Email address entered incorrectly but doesn't form a valid address -> activation email not sent
2) With UI validation:
- 2.1) Email address entered correctly -> activation email sent
- 2.2) Email address entered incorrectly but forms a valid address -> activation email sent to wrong address
- 2.3) Email address entered incorrectly but doesn't form a valid address -> user warned
-- 2.3.1) Email address re-entered correctly -> activation email sent
-- 2.3.2) other states
In the 1.3 case all of the activation emails fail to be sent. In the 2.3.1 case activation emails are sent that otherwise wouldn't be.
Sorry, yes, this wasn't supposed to be exhaustive just to illustrate the point that not providing a warning on apparently erroneous email address entries was some what pathological.
No, you're excluding the set of emails that are entered incorrectly and thus are not valid. The result for those is not the same as if the UI included a simple test (such as your "ambitious" example).
1) Without UI validation:
- 1.1) Email address entered correctly -> activation email sent
- 1.2) Email address entered incorrectly but forms a valid address -> activation email sent to wrong address
- 1.3) Email address entered incorrectly but doesn't form a valid address -> activation email not sent
2) With UI validation:
- 2.1) Email address entered correctly -> activation email sent
- 2.2) Email address entered incorrectly but forms a valid address -> activation email sent to wrong address
- 2.3) Email address entered incorrectly but doesn't form a valid address -> user warned
-- 2.3.1) Email address re-entered correctly -> activation email sent
-- 2.3.2) other states
In the 1.3 case all of the activation emails fail to be sent. In the 2.3.1 case activation emails are sent that otherwise wouldn't be.