If a user does not fill out the email address by leaving off the @ or domain - and we do not provide an example of a correct email address on submit ([email protected]) does this fail the success criterion for WCAG 2.0 3.3.3 regarding error suggestions?
I've looked at a number of major sites and leading web sites and I only see validation errors that pertain to the lack of correct email address. It looks like a lot of sites use placeholders to instruct the user on formatting, however, once submitted the user has no clear direction on proper format.
In my opinion, yes it does. This presumes that it was rejected for a known reason (formatting, etc.), so that reason should be communicated to the user.
Sadly, looking at major sites is not a good indicator of a best practice, just an indication that they are not following this checkpoint (or, more likely, are unaware of it, WCAG, and all things related).
The placeholder is never an adequate label and rarely is it adequate for instructions as it goes away. Instead, some instructional text associated with the field (maybe with ARIA if necessary) can head off errors sooner, though the error message should still convey what went wrong.
Think of all those times you enter a password only to be told after the fact that doesn't follow some arcane formatting rules. Telling you up front doesn't guarantee you'll get it right, but it reduces the likelihood everyone will get it wrong. It is also content you can re-use for the error message.