Restrict submit of form to a list of e-mail addresses

mmm interesting question @roots-of-impact . As a brute-force approach, I would explore the logic jump route, but adding a hundreds of emails is a bit dreadful. Thankfully, this can also be done programmatically fairly easily using our API:

1. add a couple of addresses in the logic builder
2. get your form’s JSON description
3. find the logic section where your list of email addresses is
4. massage the JSON payload to add the logic for each additional email address
5. update the form with the new payload from step 4. 

in a simple form example, the logic section of the form payload (expecting to filter out emails as hidden fields) would look like this:

"logic": [
        {
            "type": "hidden",
            "actions": [
                {
                    "action": "jump",
                    "details": {
                        "to": {
                            "type": "field",
                            "value": "my_perfectly_readable_reference"
                        }
                    },
                    "condition": {
                        "op": "or",
                        "vars": [
                            {
                                "op": "equal",
                                "vars": [
                                    {
                                        "type": "hidden",
                                        "value": "email"
                                    },
                                    {
                                        "type": "constant",
                                        "value": "user1@user.com"
                                    }
                                ]
                            },
                            {
                                "op": "equal",
                                "vars": [
                                    {
                                        "type": "hidden",
                                        "value": "email"
                                    },
                                    {
                                        "type": "constant",
                                        "value": "user2@user.com"
                                    }
                                ]
                            }
                        ]
                    }
                },
                {
                    "action": "jump",
                    "details": {
                        "to": {
                            "type": "field",
                            "value": "my_other_question_reference"
                        }
                    },
                    "condition": {
                        "op": "always",
                        "vars": []
                    }
                }
            ]
        }
    ],

You “just” have to repeat an “op”: “equal” block for each new email address with a quick script. 

A more polished solution @picsoung wrote about some time ago is using Serverless to prevent users from taking a survey multiple times. The concept can be reapplied to your case.

Let us know what you try and how it goes.