diff --git a/crates/kumod/src/smtp_server.rs b/crates/kumod/src/smtp_server.rs
index bab0c558..e5465185 100644
--- a/crates/kumod/src/smtp_server.rs
+++ b/crates/kumod/src/smtp_server.rs
@@ -310,6 +310,9 @@ impl SmtpServer {
             // Didn't find a complete line, fill up the rest of the buffer
             let mut data = [0u8; 1024];
             let size = self.socket.as_mut().unwrap().read(&mut data).await?;
+            if size == 0 {
+                anyhow::bail!("client disconnected");
+            }
             self.read_buffer.extend_from_slice(&data[0..size]);
         }
     }
diff --git a/crates/rfc5321/src/client.rs b/crates/rfc5321/src/client.rs
index a860450f..3ac0df1f 100644
--- a/crates/rfc5321/src/client.rs
+++ b/crates/rfc5321/src/client.rs
@@ -82,6 +82,10 @@ impl SmtpClient {
             // Didn't find a complete line, fill up the rest of the buffer
             let mut data = [0u8; MAX_LINE_LEN];
             let size = self.socket.as_mut().unwrap().read(&mut data).await?;
+            if size == 0 {
+                self.socket.take();
+                return Err(ClientError::NotConnected);
+            }
             self.read_buffer.extend_from_slice(&data[0..size]);
         }
     }