diff --git a/proton-bridge/src/api.rs b/proton-bridge/src/api.rs
new file mode 100644
index 0000000..7e1bff5
--- /dev/null
+++ b/proton-bridge/src/api.rs
@@ -0,0 +1,252 @@
+/// ProtonMail REST API client (Step 3).
+///
+/// Wraps the endpoints tuimail needs:
+///   - list messages in the inbox
+///   - fetch a single message (with encrypted body)
+///   - delete messages
+///   - fetch a recipient's public keys (for encryption when sending)
+use reqwest::Client;
+use serde::{Deserialize, Serialize};
+
+use crate::auth::Session;
+
+const API_BASE: &str = "https://mail.proton.me/api";
+
+/// ProtonMail label IDs for well-known mailboxes.
+pub const LABEL_INBOX: &str = "0";
+
+// ── API response envelopes ────────────────────────────────────────────────────
+
+/// Every ProtonMail API response wraps the payload in `{ "Code": 1000, ... }`.
+/// We only check Code in places where we want a custom error message; elsewhere
+/// `error_for_status()` on the HTTP layer is sufficient.
+#[derive(Deserialize)]
+struct Envelope<T> {
+    #[serde(rename = "Code")]
+    code: u32,
+    #[serde(flatten)]
+    data: T,
+}
+
+// ── Message list ─────────────────────────────────────────────────────────────
+
+/// Lightweight message metadata returned by `GET /mail/v4/messages`.
+#[derive(Debug, Clone, Deserialize)]
+pub struct MessageMeta {
+    #[serde(rename = "ID")]
+    pub id: String,
+    #[serde(rename = "Subject")]
+    pub subject: String,
+    /// Sender display name + address.
+    #[serde(rename = "Sender")]
+    pub sender: EmailAddress,
+    /// Unix timestamp of the message.
+    #[serde(rename = "Time")]
+    pub time: i64,
+    /// 0 = unread, 1 = read.
+    #[serde(rename = "Unread")]
+    pub unread: u32,
+    /// Message size in bytes.
+    #[serde(rename = "Size")]
+    pub size: u64,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct EmailAddress {
+    #[serde(rename = "Name")]
+    pub name: String,
+    #[serde(rename = "Address")]
+    pub address: String,
+}
+
+#[derive(Deserialize)]
+struct MessageListPayload {
+    #[serde(rename = "Messages")]
+    messages: Vec<MessageMeta>,
+    #[serde(rename = "Total")]
+    total: u32,
+}
+
+// ── Full message ──────────────────────────────────────────────────────────────
+
+/// Full message with (still-encrypted) body.
+#[derive(Debug, Clone, Deserialize)]
+pub struct Message {
+    #[serde(rename = "ID")]
+    pub id: String,
+    #[serde(rename = "Subject")]
+    pub subject: String,
+    #[serde(rename = "Sender")]
+    pub sender: EmailAddress,
+    #[serde(rename = "ToList")]
+    pub to_list: Vec<EmailAddress>,
+    #[serde(rename = "CCList")]
+    pub cc_list: Vec<EmailAddress>,
+    #[serde(rename = "Time")]
+    pub time: i64,
+    /// PGP-encrypted MIME body (decrypt in Step 4).
+    #[serde(rename = "Body")]
+    pub body: String,
+    /// MIME type of the decrypted body ("text/html" or "text/plain").
+    #[serde(rename = "MIMEType")]
+    pub mime_type: String,
+    /// The session key is encrypted with the recipient's private key.
+    #[serde(rename = "Header")]
+    pub header: String,
+}
+
+#[derive(Deserialize)]
+struct MessagePayload {
+    #[serde(rename = "Message")]
+    message: Message,
+}
+
+// ── Delete ────────────────────────────────────────────────────────────────────
+
+#[derive(Serialize)]
+struct DeleteRequest<'a> {
+    #[serde(rename = "IDs")]
+    ids: &'a [String],
+}
+
+// ── Public keys ───────────────────────────────────────────────────────────────
+
+/// One armored public key for a recipient.
+#[derive(Debug, Clone, Deserialize)]
+pub struct RecipientKey {
+    /// PGP armored public key.
+    #[serde(rename = "PublicKey")]
+    pub public_key: String,
+    /// Flags bitmap: bit 0 = can encrypt, bit 1 = can verify.
+    #[serde(rename = "Flags")]
+    pub flags: u32,
+}
+
+#[derive(Deserialize)]
+struct KeysPayload {
+    #[serde(rename = "Keys")]
+    keys: Vec<RecipientKey>,
+}
+
+// ── API client ────────────────────────────────────────────────────────────────
+
+/// Stateless API client. Pass a freshly-validated `Session` to each call.
+pub struct ApiClient<'a> {
+    client: &'a Client,
+    session: &'a Session,
+}
+
+impl<'a> ApiClient<'a> {
+    pub fn new(client: &'a Client, session: &'a Session) -> Self {
+        Self { client, session }
+    }
+
+    fn auth_headers(&self) -> [(&'static str, String); 2] {
+        [
+            ("x-pm-uid", self.session.uid.clone()),
+            ("Authorization", format!("Bearer {}", self.session.access_token)),
+        ]
+    }
+
+    /// List messages in a label (use `LABEL_INBOX`).
+    ///
+    /// Returns `(messages, total)` where `total` is the full count (for paging).
+    /// `page` is zero-based; `page_size` is typically 50.
+    pub async fn list_messages(
+        &self,
+        label_id: &str,
+        page: u32,
+        page_size: u32,
+    ) -> Result<(Vec<MessageMeta>, u32), String> {
+        let [h0, h1] = self.auth_headers();
+        let env: Envelope<MessageListPayload> = self
+            .client
+            .get(format!("{}/mail/v4/messages", API_BASE))
+            .header(h0.0, h0.1)
+            .header(h1.0, h1.1)
+            .query(&[
+                ("LabelID", label_id),
+                ("Page", &page.to_string()),
+                ("PageSize", &page_size.to_string()),
+                ("Sort", "Time"),
+                ("Desc", "1"),
+            ])
+            .send()
+            .await
+            .map_err(|e| e.to_string())?
+            .error_for_status()
+            .map_err(|e| e.to_string())?
+            .json()
+            .await
+            .map_err(|e| e.to_string())?;
+
+        if env.code != 1000 {
+            return Err(format!("list_messages: API code {}", env.code));
+        }
+        Ok((env.data.messages, env.data.total))
+    }
+
+    /// Fetch a single message with its (encrypted) body.
+    pub async fn get_message(&self, id: &str) -> Result<Message, String> {
+        let [h0, h1] = self.auth_headers();
+        let env: Envelope<MessagePayload> = self
+            .client
+            .get(format!("{}/mail/v4/messages/{}", API_BASE, id))
+            .header(h0.0, h0.1)
+            .header(h1.0, h1.1)
+            .send()
+            .await
+            .map_err(|e| e.to_string())?
+            .error_for_status()
+            .map_err(|e| e.to_string())?
+            .json()
+            .await
+            .map_err(|e| e.to_string())?;
+
+        if env.code != 1000 {
+            return Err(format!("get_message: API code {}", env.code));
+        }
+        Ok(env.data.message)
+    }
+
+    /// Move messages to Trash (ProtonMail's delete is a soft-delete to Trash).
+    pub async fn delete_messages(&self, ids: &[String]) -> Result<(), String> {
+        let [h0, h1] = self.auth_headers();
+        self.client
+            .put(format!("{}/mail/v4/messages/delete", API_BASE))
+            .header(h0.0, h0.1)
+            .header(h1.0, h1.1)
+            .json(&DeleteRequest { ids })
+            .send()
+            .await
+            .map_err(|e| e.to_string())?
+            .error_for_status()
+            .map_err(|e| e.to_string())?;
+        Ok(())
+    }
+
+    /// Fetch the public keys for a recipient email address.
+    /// Used when composing outbound mail (Step 7).
+    pub async fn get_public_keys(&self, email: &str) -> Result<Vec<RecipientKey>, String> {
+        let [h0, h1] = self.auth_headers();
+        let env: Envelope<KeysPayload> = self
+            .client
+            .get(format!("{}/core/v4/keys", API_BASE))
+            .header(h0.0, h0.1)
+            .header(h1.0, h1.1)
+            .query(&[("Email", email)])
+            .send()
+            .await
+            .map_err(|e| e.to_string())?
+            .error_for_status()
+            .map_err(|e| e.to_string())?
+            .json()
+            .await
+            .map_err(|e| e.to_string())?;
+
+        if env.code != 1000 {
+            return Err(format!("get_public_keys: API code {}", env.code));
+        }
+        Ok(env.data.keys)
+    }
+}
diff --git a/proton-bridge/src/main.rs b/proton-bridge/src/main.rs
index d9893ca..9f414b6 100644
--- a/proton-bridge/src/main.rs
+++ b/proton-bridge/src/main.rs
@@ -1,7 +1,10 @@
+mod api;
 mod auth;
 mod config;
 mod srp;
 
+use api::{ApiClient, LABEL_INBOX};
+
 #[tokio::main]
 async fn main() {
     let config = match config::Config::load() {
@@ -14,8 +17,26 @@ async fn main() {
         Err(e) => { eprintln!("Failed to build HTTP client: {}", e); std::process::exit(1); }
     };
 
-    match auth::authenticate(&client, &config.proton).await {
-        Ok(session) => println!("Session UID: {}", session.uid),
+    let session = match auth::authenticate(&client, &config.proton).await {
+        Ok(s) => s,
         Err(e) => { eprintln!("Authentication failed: {}", e); std::process::exit(1); }
+    };
+    println!("Session UID: {}", session.uid);
+
+    // Step 3 smoke-test: list the first page of inbox messages.
+    let api = ApiClient::new(&client, &session);
+    match api.list_messages(LABEL_INBOX, 0, 10).await {
+        Ok((messages, total)) => {
+            println!("\nInbox ({total} total):");
+            for m in &messages {
+                println!(
+                    "  [{:>5}] {:50} — {}",
+                    if m.unread == 1 { "UNREAD" } else { "  read" },
+                    m.subject,
+                    m.sender.address,
+                );
+            }
+        }
+        Err(e) => eprintln!("list_messages failed: {}", e),
     }
 }
\ No newline at end of file