The title pretty much says it all, but for those wanting a bit more context: in our specific use case, the back-end API must reply to some HTTP GET requests with a redirection to an email address (with the typical purpose of automatically opening the end-user's email client).
For those wondering why we don't simply link to the actual email address directly in the UI, the reason is that the specific email address is unknown at the time the UI is displayed because it varies depending on the specifics of the request. And also, because the front-end must not rely on JavaScript, this must therefore be handled by the back-end.
So, should this redirection be a 303/seeother? I have read this Wikipedia page but am still unsure whether a mailto: URL qualifies as a resource for which "the origin server does not have a representation".
(Is this the correct SE site for this question or would it go better on Stack Overflow?)