ability to overwrite the host which is used to fetch mail content

[Nemo64]

I have multiple hosting situations where I can't create request on the website domain or localhost directly. One is my local docker development environment and the other is a live website behind a load balancer and a very strict firewall.

What i'm proposing is a global configuration like "overwrite content fetch host" where I can define which host actually works from the php processes perspective. I'd also send the correct Host: header to fake the domain which should actually make the request.

Since this is a very specific problem (which i happen to have multiple times) I volunteer to implement it. I just don't have the time right now and maybe someone else has this issue too so it can be tracked here.