Would an app only accessible to existing customers pass the App Store review?

I believe it would not be acceptable to require login in order to provide any functionality. The app has to do at least something for anyone running it. For those users who satisfy your customer status it can do more. Also - you cannot unlock code for your customers but you can provide a 'reader' app that displays content purchased outside the app.

>the app would only be a way to access the same content

In that case, according to the ASRGs, Section 4 Design / 4.2 Minimum Functionality seems clear enough - it would risk easy rejection.

Like the OP, I also want to publish an app that provides certain information.

If you're not logged in, you can also access the information, however it would be limited to information three days old (and older).

In order to get current information you need to log in.

My question is: Can I provide an input box for a 'customer key' which would behave very much like a license key? Or do I have to provide a full login using email and password?

Story behind is: Each of our customer has a list of 'customer keys' which they can give to their workers (who download and activate the app). Without a license key input, all workers must have access to the company's account information (email and password) which I believe to be inacceptable.

>Each of our customer has a list of 'customer keys' which they can give to their workers (who download and activate the app). Without a license key input, all workers must have access to the company's account information (email and password) which I believe to be inacceptable.

Your question is about workers/employees, so you may want to start your own thread, perhaps in the Enterprise forum.

That said, Apple's Volume Purchase Program involves purchase of blocks of redeem codes that are then distributed for download from the store.

What you want to do violates guideline 3.1.4. Apple has no way of knowing whether you are unlocking parts of your app in exchange for payments outside of the IAP system. If you are only providing a 'reader' app (3.1.3) then you can do what you are proposing.

An app dedicated to existing customers sounds like most major banking apps (many in the App store).

So, if we have a "reader" app, allowing our existing customers to access their purchased content when logging in using their credentials (email + password), it should be ok, right?

It would be ok - but the app also has to do something, anything, for users who have not purchased content from your website to 'read' in the app. Perhaps explain how they can go to your website to select content. But don't violate the restrictions on reader apps in the guidelines.

Regarding 3.1.4 Content Codes:

[...] App features that work in combination with an approved physical product [...] on an optional basis [...]

What exactely is an approved physical product?

This is for something like a barcode reader (or other equipment) that attaches to the phone, it may need an app or some software to control it.

If it’s not acceptable then how are apps like Salesforce and Insightly allowed on the App Store? When you download them and open them up all you see is a login page. You can’t create an account nor is there any information linking you to where you can.