Piwigo has an S3 extension https://piwigo.org/ext/extension_view.php?eid=691

I agree that is sucks but my advice is purely practical. I try to be principled where I can but email is a lost cause in my opinion. I have hosted my own Mailserver. I encountered providers that only have allow lists and needed me to manually apply to allow my mail in. You have to build a reputation to not be marked as spam. Someone can just submit your domain to a block list and you have to find out how to get removed. It is a mess.

The cold hard truth is that if you want your mail delivered reliably, don’t host yourself.

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The biggest problem is not getting your application to send mail, but to get the mail into the inboxes of people and not spam. That is what you pay the big providers for. Email is broken in that way. I’d advise you to go for a paid service if you want your emails to reliably arrive.

What about mDNS?

Doing these “find your device with magic and do stuff” things can be a bit troublesome across networks. Some is possible to set up but sometimes it just doesn’t work. It is the tradeoff between security and comfort.

It depends on what you want to do with it. WebDAV might be an idea.

I don’t think that is a good idea. NFS is more designed for LAN than for WAN.

I did host my email, but the problem wasn’t the spam but the bigger email providers. Best case was my mail was marked as spam. Worst case was that I was blocked until I jumped through hoops. Email hosting is unfortunately broken.

Is it possible you have a faulty disk?