diff --git a/src/api/openapi.rs b/src/api/openapi.rs
index bc2ded3..5bbfdd3 100644
--- a/src/api/openapi.rs
+++ b/src/api/openapi.rs
@@ -56,6 +56,7 @@ use utoipa::OpenApi;
         crate::api::wallet::grin::scan,
         crate::api::wallet::grin::height,
         crate::api::wallet::grin::broadcast,
+        crate::api::wallet::grin::address_user,
         crate::api::wallet::grin_relay::create,
         crate::api::wallet::grin_relay::pending,
         crate::api::wallet::grin_relay::get_relay,
@@ -144,6 +145,7 @@ use utoipa::OpenApi;
         crate::api::wallet::grin::GrinHeightResponse,
         crate::api::wallet::grin::GrinBroadcastRequest,
         crate::api::wallet::grin::GrinBroadcastResponse,
+        crate::api::wallet::grin::GrinAddressUserResponse,
         crate::api::wallet::grin_relay::CreateRelayRequest,
         crate::api::wallet::grin_relay::RespondRelayRequest,
         crate::api::wallet::grin_relay::FinalizeRelayRequest,
diff --git a/src/api/wallet/grin.rs b/src/api/wallet/grin.rs
index fa3c613..6fe45a4 100644
--- a/src/api/wallet/grin.rs
+++ b/src/api/wallet/grin.rs
@@ -17,7 +17,7 @@
 use std::sync::Arc;
 
 use axum::{
-    extract::State,
+    extract::{Path, State},
     http::HeaderMap,
     routing::{get, post},
     Json, Router,
@@ -93,6 +93,31 @@ pub struct GrinBroadcastResponse {
     pub ok: bool,
 }
 
+/// Resolution of a bare Grin address to its owning user's routing identity.
+///
+/// The address→npub **bridge**: given a `grin1…` slatepack address, report
+/// whether it belongs to a user registered on *this* backend and, if so, that
+/// user's linked Nostr pubkey so a bare-address send can be upgraded to a
+/// federated Nostr gift-wrap instead of the same-instance relay.
+///
+/// NOTE (federation): this is a **same-instance convenience**. It can only
+/// resolve addresses whose owner registered their Grin key here (via `POST
+/// /keys`). It is *not* a federated directory — a wallet on another backend
+/// won't be found. `registered: false` (a constant-shape miss, mirroring
+/// `GET /users/by-username`) means "unknown to this instance", at which point
+/// the sender falls back to manual clipboard / the backend relay.
+#[derive(Debug, Serialize, utoipa::ToSchema)]
+pub struct GrinAddressUserResponse {
+    /// Whether the address resolved to a user on this backend.
+    pub registered: bool,
+    /// The owning user's id (UUID), when `registered`.
+    pub user_id: Option<String>,
+    /// The owning user's linked Nostr pubkey (x-only hex), when they have one.
+    /// This is what the sender feeds the Nostr gift-wrap channel to route the
+    /// slate over Nostr. `None` when the user registered no Nostr identity.
+    pub npub: Option<String>,
+}
+
 fn output_dto(o: ViewWalletOutputResult) -> GrinOutput {
     GrinOutput {
         commit: o.commit,
@@ -200,6 +225,46 @@ pub async fn broadcast(
     Ok(Json(GrinBroadcastResponse { ok: true }))
 }
 
+/// Resolve a bare Grin address to its owning user's Nostr routing identity.
+///
+/// The address→npub bridge (see [`GrinAddressUserResponse`]). JWT-gated: only
+/// authenticated senders resolve routing. An address unknown to this backend
+/// returns the constant-shape `registered: false` response — the caller then
+/// falls back to manual clipboard or the same-instance relay.
+#[utoipa::path(
+    security(("bearer_auth" = [])),
+    get,
+    path = "/wallet/grin/address/{addr}/user",
+    params(("addr" = String, Path, description = "Grin address (registered public key)")),
+    responses(
+        (status = 200, description = "Address resolution result", body = GrinAddressUserResponse),
+        (status = 401, description = "Missing or invalid token")
+    ),
+    tag = "grin"
+)]
+#[instrument(skip(state, headers))]
+pub async fn address_user(
+    State(state): State<Arc<AppState>>,
+    headers: HeaderMap,
+    Path(addr): Path<String>,
+) -> Result<Json<GrinAddressUserResponse>, AppError> {
+    extract_user_id_from_token(&state, &headers).await?;
+
+    let Some(user) = state.db.find_user_by_grin_address(&addr).await? else {
+        return Ok(Json(GrinAddressUserResponse {
+            registered: false,
+            user_id: None,
+            npub: None,
+        }));
+    };
+
+    Ok(Json(GrinAddressUserResponse {
+        registered: true,
+        user_id: Some(user.id.to_string()),
+        npub: user.nostr_pubkey,
+    }))
+}
+
 // ── router ────────────────────────────────────────────────────────────────────
 
 /// Grin routes, RELATIVE to the `/api/v1` mount point.
@@ -208,4 +273,5 @@ pub fn routes() -> Router<Arc<AppState>> {
         .route("/wallet/grin/scan", post(scan))
         .route("/wallet/grin/height", get(height))
         .route("/wallet/grin/broadcast", post(broadcast))
+        .route("/wallet/grin/address/:addr/user", get(address_user))
 }
diff --git a/src/api/wallet/grin_relay.rs b/src/api/wallet/grin_relay.rs
index 23ab174..2524747 100644
--- a/src/api/wallet/grin_relay.rs
+++ b/src/api/wallet/grin_relay.rs
@@ -19,6 +19,23 @@
 //! must be a registered user (no blind spam to arbitrary addresses); slatepack
 //! sizes are capped; entries expire; and the whole feature is behind
 //! `FEATURE_GRIN_RELAY` so an operator can disable the mailbox.
+//!
+//! ## Federation: this relay is a SAME-INSTANCE CONVENIENCE, not the default
+//!
+//! The relay only connects two users **registered on the same backend**: the
+//! sender is the JWT caller and the recipient is addressed by a
+//! `recipient_user_id` that must already exist in *this* instance's `users`
+//! table (there is deliberately no cross-instance address lookup here). A wallet
+//! on another backend cannot be a relay counterparty.
+//!
+//! It is therefore NOT the federated transport. The **default, federated**
+//! transport for interactive Grin transfers is Nostr gift-wrap (npub / NIP-05),
+//! which works across instances; **manual clipboard** is the universal fallback.
+//! Prefer those. This mailbox is offered only as an ergonomic shortcut when both
+//! parties happen to live on the same instance — treat it as an optimization,
+//! never as something a second backend can rely on. See the address→npub bridge
+//! in [`super::grin`] (`/wallet/grin/address/{addr}/user`), which lets a
+//! same-instance address be upgraded to federated Nostr routing.
 
 use std::sync::Arc;
 
diff --git a/src/infra/db/users.rs b/src/infra/db/users.rs
index 60b090c..f127efa 100644
--- a/src/infra/db/users.rs
+++ b/src/infra/db/users.rs
@@ -222,6 +222,29 @@ impl Database {
             .await?)
     }
 
+    /// Resolve a registered Grin address to its owning user.
+    ///
+    /// The `address` is matched verbatim against `user_keys.public_key` for the
+    /// `grin` asset (the canonical form the wallet registered via `POST /keys`).
+    /// This is a **same-instance** bridge: it only resolves addresses whose owner
+    /// registered on *this* backend. Used to look up a recipient's linked Nostr
+    /// pubkey so a bare-`grin1…`-addressed send can route over Nostr gift-wrap.
+    #[instrument(skip(self, address))]
+    pub async fn find_user_by_grin_address(
+        &self,
+        address: &str,
+    ) -> Result<Option<User>, AppError> {
+        let sql = format!(
+            "SELECT {USER_COLS} FROM users u \
+             JOIN user_keys k ON k.user_id = u.id \
+             WHERE k.asset = 'grin' AND k.public_key = $1"
+        );
+        Ok(sqlx::query_as::<_, User>(&sql)
+            .bind(address)
+            .fetch_optional(self.pool())
+            .await?)
+    }
+
     #[instrument(skip(self))]
     pub async fn get_user_by_username(&self, username: &str) -> Result<Option<User>, AppError> {
         let sql = format!("SELECT {USER_COLS} FROM users WHERE username = $1");