Hey,
I would've thought you can use jsonpath regex match assertion if you want to use the OTB functionality - you just need to determine the regex for your email address - email addresses are a littel awkward with regex - there's lots of different options trying to cover all permutations.
Found via link
Here's one --> ^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$
And this covers the following: Doesn't allow numbers in the domain name and doesn't allow for top level domains that are less than 2 or more than 3 letters (which is fine until they allow more). Doesn't handle multiple "." in the domain (joe@abc.co.uk). If the 'allowed' email addresses are different - tailor the above regex accordingly
If you just need literally something to verify that the @ is in the email address and that the email address is comprised of up to 255 chars you could try the following regex (which I've just put together now) [a-z][A-Z][0-9]{250}@.[a-z][A-Z]{3} - this is a lot more basic than the above regex from the link - but it might do the job.
I suppose as you're using a script, you might want to add the assertion into your script - with this in mind, please see the following stackoverflow link which has details on asserting regex values
I'm not a scripter - so I can't give you the code - but hopefully the details/links above might help get you there
hope the above helps,
richie