diff --git a/internal/client/smtpimap/imap/err.go b/internal/client/smtpimap/imap/err.go
index 53c0aaa3..e4e41ab2 100644
--- a/internal/client/smtpimap/imap/err.go
+++ b/internal/client/smtpimap/imap/err.go
@@ -2,6 +2,8 @@ package imap
 
 import (
 	"errors"
+	"fmt"
+	"strings"
 
 	"github.com/emersion/go-imap/v2"
 	"github.com/warmbly/warmbly/internal/errx"
@@ -21,14 +23,7 @@ func (c *Client) handleError(err error) *errx.MailError {
 		case imap.ResponseCodeAuthorizationFailed:
 			return errx.ErrMailAuthorizationFailed
 		default:
-			// A NO/BAD without a response code (Gmail's "NO System Error")
-			// has only its text; an empty code rendered as "Something went
-			// wrong: " in the mailbox's error list.
-			detail := string(imapErr.Code)
-			if detail == "" {
-				detail = imapErr.Text
-			}
-			return errx.ErrMailUnknownImapError(detail)
+			return errx.ErrMailUnknownImapError(imapErrDetail(imapErr))
 		}
 	}
 
@@ -44,3 +39,20 @@ func (c *Client) handleError(err error) *errx.MailError {
 	// pass instead of deactivating the mailbox.
 	return errx.ErrMailServerUnreachable
 }
+
+// imapErrDetail is the part of the mailbox's error row that says what the
+// server actually refused. The response code is optional in IMAP, and a
+// codeless NO/BAD (IONOS, Gmail's "NO System Error") rendered as "Something
+// went wrong: " with nothing after the colon, which is unactionable for the
+// customer and undiagnosable from a bug report. Never returns "".
+func imapErrDetail(err *imap.Error) string {
+	if err.Code != "" {
+		return string(err.Code)
+	}
+	// Keep the status: a BAD means we sent something the server does not
+	// understand, a NO means it understood and declined.
+	if detail := strings.TrimSpace(fmt.Sprintf("%s %s", err.Type, err.Text)); detail != "" {
+		return detail
+	}
+	return "the mail server refused the command without saying why"
+}
diff --git a/internal/client/smtpimap/imap/err_test.go b/internal/client/smtpimap/imap/err_test.go
index 6eb719a4..694f46e8 100644
--- a/internal/client/smtpimap/imap/err_test.go
+++ b/internal/client/smtpimap/imap/err_test.go
@@ -3,8 +3,10 @@ package imap
 import (
 	"io"
 	"net"
+	"strings"
 	"testing"
 
+	goimap "github.com/emersion/go-imap/v2"
 	"github.com/warmbly/warmbly/internal/errx"
 )
 
@@ -26,3 +28,65 @@ func TestHandleErrorTransportIsNotNil(t *testing.T) {
 		t.Error("handleError(nil) must stay nil")
 	}
 }
+
+// A NO/BAD carries a response code only when the server chooses to send one.
+// A codeless one used to render as "Something went wrong: " with nothing after
+// the colon (issue #405, IONOS), which tells the customer nothing and leaves a
+// bug report with no way to identify the refused command.
+func TestHandleErrorCodelessImapErrorKeepsServerText(t *testing.T) {
+	c := &Client{}
+
+	for _, tc := range []struct {
+		name string
+		err  *goimap.Error
+		want string
+	}{
+		{
+			name: "codeless NO keeps the server's text",
+			err:  &goimap.Error{Type: goimap.StatusResponseTypeNo, Text: "System Error"},
+			want: "NO System Error",
+		},
+		{
+			name: "codeless BAD is distinguishable from a NO",
+			err:  &goimap.Error{Type: goimap.StatusResponseTypeBad, Text: "Command unrecognized"},
+			want: "BAD Command unrecognized",
+		},
+		{
+			name: "a response code still wins over the text",
+			err:  &goimap.Error{Type: goimap.StatusResponseTypeNo, Code: goimap.ResponseCodeServerBug, Text: "oops"},
+			want: "SERVERBUG",
+		},
+		{
+			name: "no code and no text still says something",
+			err:  &goimap.Error{Type: goimap.StatusResponseTypeNo},
+			want: "NO",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			got := c.handleError(tc.err)
+			if got == nil {
+				t.Fatalf("handleError(%v) = nil, want a mail error", tc.err)
+			}
+			if got.Code != errx.MailErrorCodeImapUnknown {
+				t.Fatalf("Code = %q, want %q", got.Code, errx.MailErrorCodeImapUnknown)
+			}
+			if !strings.Contains(got.Message, tc.want) {
+				t.Errorf("Message = %q, want it to contain %q", got.Message, tc.want)
+			}
+		})
+	}
+}
+
+// The whole point of the fallback: the detail is never empty, so the row can
+// never read as a bare "Something went wrong: " again.
+func TestHandleErrorImapDetailIsNeverEmpty(t *testing.T) {
+	for _, err := range []*goimap.Error{
+		{},
+		{Type: goimap.StatusResponseTypeNo},
+		{Type: goimap.StatusResponseTypeBad, Text: "   "},
+	} {
+		if detail := imapErrDetail(err); strings.TrimSpace(detail) == "" {
+			t.Errorf("imapErrDetail(%+v) = %q, want a non-empty detail", err, detail)
+		}
+	}
+}
diff --git a/web/src/components/app/unibox/ThreadView.tsx b/web/src/components/app/unibox/ThreadView.tsx
index 8e55b361..641928b3 100644
--- a/web/src/components/app/unibox/ThreadView.tsx
+++ b/web/src/components/app/unibox/ThreadView.tsx
@@ -160,11 +160,26 @@ export function ThreadView({ threadId, emailId }: ThreadViewProps) {
   const threadLabels = useThreadLabels(threadId);
   const [labelMenuOpen, setLabelMenuOpen] = React.useState(false);
 
-  // CRM context rail (right side). Starts closed at every width and is
-  // opened from the header toggle: on wide screens it renders as a static
-  // rail, below lg as an overlay drawer. It used to open by itself on lg+,
-  // which put the contact form in front of every thread the reader opened.
-  const [crmOpen, setCrmOpen] = React.useState(false);
+  // CRM context rail (right side). Open by default on lg+, where it renders
+  // as a static rail beside the thread; below lg it is an overlay drawer, so
+  // it starts closed and is opened from the header toggle.
+  const [crmOpen, setCrmOpen] = React.useState(
+    () =>
+      typeof window !== "undefined" &&
+      window.matchMedia("(min-width: 1024px)").matches,
+  );
+
+  // The initial state is read once, so narrowing past lg with the rail open
+  // turned it into an overlay sitting on top of the thread (a rotated tablet,
+  // a window dragged to half a screen). Close it on the way down.
+  React.useEffect(() => {
+    const mq = window.matchMedia("(min-width: 1024px)");
+    const onChange = (e: MediaQueryListEvent) => {
+      if (!e.matches) setCrmOpen(false);
+    };
+    mq.addEventListener("change", onChange);
+    return () => mq.removeEventListener("change", onChange);
+  }, []);
 
   // `c` opens the label menu while a thread is open — ignored while
   // typing into the composer / any input so it never eats keystrokes.