fix(smtp): close the raw email stream when a relay does not send it (#423)

relayEmail opens a read stream on the .eml file for every relay, and only
client.send() ever consumed it. When the auto-relay rules left no
recipients, or the outgoing server failed the send (e.g. rejected a
recipient), the stream was never read or destroyed and its file
descriptor stayed open. Destroy it on the early return and in the finally
block; after a successful send it is already fully read, so that is a
no-op.

packages/smtp/src/relay.ts

@@ -100,6 +100,7 @@ export class RelayClient {
 
     // Fail silently if no recipients
     if (recipients.length === 0) {
+      emailStream.destroy()
       return {
         success: false,
         message: 'Email had no recipients after filtering',
@@ -132,6 +133,9 @@ export class RelayClient {
         error: err,
       }
     } finally {
+      // The stream holds an open file descriptor until it is fully read or
+      // destroyed; on a failed relay it was never (fully) piped to the server.
+      emailStream.destroy()
       await this.disconnect()
     }
   }