is not a valid email address in the basic format local-part@hostname".
A simple way to do this is to use the setMessage() method of the Zend_Validate_EmailAddress. Here's my solution below:
$email = new Zend_Form_Element_Text('Email');
$email->getValidator('EmailAddress')->setMessage('Ooops, %value% is wrong!!!', Zend_Validate_EmailAddress::INVALID_FORMAT);
In the code above, I have set the EmailAddress (Zend_Validate_EmailAddress) validator as a validator of my form's email element. In order to mess with the message, we need to retrieve the validator object by using the getValidator method of the Zend_Form_Element. From here you can access the set method setMessage and all you have to do is replace it with your new message, and target the message key which is Zend_Validate_EmailAddress::INVALID_FORMAT. Remember that setMessage overrides the default value of the INVALID constant.
Thats pretty much it!