fix: Still try to create "INBOX.DeltaChat" if couldn't create "DeltaChat" (#5870)

It appeared that some servers require namespace-style names for folders created via IMAP, like
"INBOX.DeltaChat". This partially reverts 05c256dd5b.

src/imap.rs

@@ -1510,8 +1510,8 @@ impl Session {
 
     /// Attempts to configure mvbox.
     ///
-    /// Tries to find any folder in the given list of `folders`. If none is found, tries to create
+    /// Tries to find any folder examining `folders` in the order they go. If none is found, tries
-    /// `folders[0]`. This method does not use LIST command to ensure that
+    /// to create any folder in the same order. This method does not use LIST command to ensure that
     /// configuration works even if mailbox lookup is forbidden via Access Control List (see
     /// <https://datatracker.ietf.org/doc/html/rfc4314>).
     ///
@@ -1545,9 +1545,9 @@ impl Session {
         if !create_mvbox {
             return Ok(None);
         }
-        let Some(folder) = folders.first() else {
+        // Some servers require namespace-style folder names like "INBOX.DeltaChat", so we try all
-            return Ok(None);
+        // the variants here.
-        };
+        for folder in folders {
             match self.select_with_uidvalidity(context, folder).await {
                 Ok(_) => {
                     info!(context, "MVBOX-folder {} created.", folder);
@@ -1557,6 +1557,7 @@ impl Session {
                     warn!(context, "Cannot create MVBOX-folder {:?}: {}", folder, err);
                 }
             }
+        }
         Ok(None)
     }
 }