fix: In SMTP loop, reset transport_id and from when disconnecting (#8797)

Previously, when we disconnected from the current transport, we forgot
to reset the `transport_id` and `from`. I couldn't find any bug that is caused by
this today, but it was a lingering problem.

src/smtp.rs

@@ -68,6 +68,8 @@ impl Smtp {
             // separate task to avoid waiting for reply or timeout.
             task::spawn(async move { transport.quit().await });
         }
+        self.transport_id = None;
+        self.from = None;
         self.last_success = None;
     }