A simple approach would be to just run an IMAP server. For receiving you use a script using POP3 to download newly arrived emails (which is run every 5 minutes for example). If your server is offline, nothing gets lost.
For the sending side you can either use your mail ISP’s SMTP server directly from the client or you install a local relay as a mitm to the ISP’s SMTP.
CF is not using „their own“! The certificates the client see must be provided and authorized by the provider of the service. Or put in other words: CF is acting as the hosting provider to the outside, to the clients.
The rest of journey is „inside“ the domain of the provider of the service. It is totally normal that traffic has some journey to go and often it never touches the premises of the provider or even a server owned by the provider.
The important thing that all the part which from a customer‘s view is „internal to the provider of the service“ (behind the CF address) is responsibility of the provider of the service, no matter what 3rd party services they use.
Are you sending lots of mails to a large group of users. If not why not use any normal e-mail service like gmail?
Use an outbound tunnel like the others recommended already.
For curiosity: a mobile hotspot in my mind is a device connecting to Internet via mobile data (LTE, 5G etc) and offering a WLAN. In this case this is your router. But mobile provider often offer no public reachability at all - in which case you are back to the tunnel.
CF Tunnels. Based on a reverse proxy in the cloud with a VPN between local and CF. So different from a direct IP connection.
Or: IPv6 could be a way out.
Do you use it with a single network port or do you add interfaces, via USB? Wasn’t there a performance limitation at least on the USB ports? Maybe the 5 has better specs here?
Did you look into GL.inet routers? The have OpenWRT under the hood? At their web shop cheaper than at Amazon.
Is this your LAN subnet? It looks very much as a docker virtual network? These addresses shouldn‘t be accessible from outside unless you tinker with the network settings of docker. Containers should be accessible on localhost:externalPort from the host docker is running on.
No. This is the text I am referring to: „via http://internal-docker-IP:port. but picsur doesnt work via that way. Connection refused. so http://internal-docker-IP:8044/ (8077 also doesnt work)“
I am unsure if I decode this meta information correctly.
(Reddit drives me crazy: my answers appear on top level when I want to reply in thread and vice versa)
Don‘t know picsur. But first step the service should work without reverse proxy. So try from docker host, then from same subnet.
Also the directive networks inside of the sb container makes no sense. Delete both lines.
After this look into the nginx log files (access and error)!
Run it on each docker host and combine the output by creating a simple html frame around it, like an iframe per instance and displaying it either side by side or as „tabs“ to click through?