Calling it a day...

client.go

@@ -295,6 +295,18 @@ func (c *Client) Close() error {
 	return nil
 }
 
+// Reset sends the RSET command to the SMTP client
+func (c *Client) Reset() error {
+	if err := c.checkConn(); err != nil {
+		return err
+	}
+	if err := c.sc.Reset(); err != nil {
+		return fmt.Errorf("failed to send RSET to SMTP client: %w", err)
+	}
+
+	return nil
+}
+
 // DialAndSend establishes a connection to the SMTP server with a
 // default context.Background and sends the mail
 func (c *Client) DialAndSend() error {
@@ -314,6 +326,9 @@ func (c *Client) checkConn() error {
 	if c.co == nil {
 		return ErrNoActiveConnection
 	}
+	if err := c.sc.Noop(); err != nil {
+		return ErrNoActiveConnection
+	}
 	if err := c.co.SetDeadline(time.Now().Add(c.cto)); err != nil {
 		return ErrDeadlineExtendFailed
 	}

cmd/main.go

@@ -41,6 +41,10 @@ func main() {
 		os.Exit(1)
 	}
 	defer func() {
+		if err := c.Reset(); err != nil {
+			fmt.Printf("failed to reset: %s\n", err)
+			os.Exit(1)
+		}
 		if err := c.Close(); err != nil {
 			fmt.Printf("failed to close: %s\n", err)
 			os.Exit(1)