accounts.email: add mailbox.org flavor

Based on the [official documentation][1].

[1]: https://kb.mailbox.org/en/private/e-mail/e-mail-configuration/

modules/accounts/email.nix

@@ -369,6 +369,7 @@ let
             "davmail"
             "fastmail.com"
             "gmail.com"
+            "mailbox.org"
             "migadu.com"
             "outlook.office365.com"
             "plain"
@@ -592,6 +593,13 @@ let
           };
         })
 
+        (mkIf (config.flavor == "mailbox.org") {
+          userName = mkDefault config.address;
+          folders.inbox = mkDefault "INBOX";
+          imap.host = "imap.mailbox.org";
+          smtp.host = "smtp.mailbox.org";
+        })
+
         (mkIf (config.flavor == "migadu.com") {
           userName = mkDefault config.address;