ASP.NET OnClientClick="return false;" doesn't work

for some reason, although I didn't have any jquery event handlers attached, it didn't work.

What actually worked was:

OnClientClick="if (validate_form() == false) return(false);"

Do you have a click event handler (registered via jquery) which returns true? In that case, the return value of OnClientClick is ignored.

Have a look at my question (and answer): Why doesn't returning false from OnClientClick cancel the postback

Try changing it to

OnClientClick="ValidateMail(); return false;"