Describe the problem you are trying to fix (provide as much context as possible)

Try to validate more than one email in a string that separate by comma, it throw exception.
Now for most part, the multiple email address is probably supported.
EX. abc@aaa.com, bbb@ccc.com

Which API (or modification of the current API) do you suggest to solve that problem ?

joi.string().email()

// maybe
joi.string().email({
    multiple: true,
})

Are you ready to work on a pull request if your suggestion is accepted ?

Yes.