validateemailaddress() ?

As this is no function of the Appian product, you would need to check the code.

you can get the list of valid emails from google http://softwaretesterfriend.com/manual-testing/valid-invalid-email-address-format-validation/

Tried some based on the valid email format mentioned in the site for some it didnt work.

nice try...

No all the cases will not be covered using that function. 

You can use the below regex expression to check that 

for validating email address you can use this regex 

regexmatch("^[\w\.\-]+@[\w\.\-]+\.[\w\.]{2,}$",ri!input)

This pattern fails a bunch of the test cases I have set up in my Validate Email Addresses expression rule.  Two of the most obvious/important failure cases is that it doesn't return "false" for email addresses beginning or ending with extraneous periods (".").

This is the pattern I use in mine, which passes a rather rigorous 18 test cases (though i can't guarantee it passes all realistic ones, just the ones i've been able to think of).

regexmatch(
  pattern: "^[A-Z0-9\'\+\&\%_-]+(\.{0,1}[A-Z0-9\'\+\&\%_-]+)*[@]{1}[A-Z0-9.-]*[A-Z0-9-]+[.]{1}[A-Z]{2,6}$",
  searchString: ri!email,
  regexFlags: "si"
)

Also note: all of the above test cases also correctly pass using the "validateEmailAddress()" function from People Functions, for what it's worth.

Is there a way to display a message instead of a boolean value with this?

You would put the message at the parent (interface) level when this rule returns a FALSE value, for instance.

But really, it's an expression rule, you can make yours return any value you want - that's a pretty basic change.