When validating email address on sign-up/login page which is the right grammar?
"Please insert a valid email address."
"Please enter a valid email address."
"Invalid email address." like the Google does.
Provide direct, simple feedback that people can understand. For example, error messages should spell out exactly what situation caused the error (“There’s not enough space on that disk to save the document”) and possible actions the user can take to rectify it (“Try saving the document in another location”).
Effective error messages inform users that a problem occurred, explain why it happened, and provide a solution so users can fix the problem. Users should either perform an action or change their behavior as the result of an error message.
which, to me, makes a lot of sense... so i'd go with something like "Invalid email address. Valid e-mail can contain only latin letters, numbers, '@' and '.'", or something more appropriate to your validations...
IMHO what matters most in feedback messages, like error, success, info, and warning messages, are the following items:
Now, if you have all of the other items in place, I think message should be as human-understandable as it can, but at the same time, as short as it can. Directive-like messages can be a good choice, because they both end as a usual sentence, i.e. they're not a phrasal sentence, and they are also more fluent for users to read. Thus, I think Please enter a valid email address. would be the best fit. Also you might become more explicit by showing The email address is not valid. or Please enter a valid email address.
Also I have captured the Firefox message, for an invalid email address for
<input type='email' /> field.
The problem with "Please insert/enter a valid email address." is that the focus doesn’t lie on the valid part of this message. It’s easy to overlook it, especially when only the first part of the message is read (who likes to read error messages?).
Users might expect such a message also in case they forgot to enter an email address. So they might think "Huh? I did enter my email address!".
So I think it’s better to put the actual problem in focus, and at best near the beginning of the message.
According to this, "Invalid email address." should work pretty well. It’s short (hard not to read the full message), and the actual problem is the first word ("Invalid").
Of course, if your system knows why the email address is invalid, you should include the reason in the message, too (e.g., in a second short sentence).