Chore: Refactor error handling and resource management across multiple files (golangci-lint)

- Updated error handling to use the error return value for resource closures in tests and functions, ensuring proper error reporting.
- Replaced direct calls to `Close()` with deferred functions that handle errors gracefully.
- Improved readability by using `strings.ReplaceAll` instead of `strings.Replace` for string manipulation.
- Enhanced network connection handling by adding default cases for unsupported network types.
- Updated HTTP response handling to use the appropriate status codes and error messages.
- Removed unused variables and commented-out code to clean up the codebase.

sendmail/cmd/cmd.go

@@ -121,14 +121,14 @@ func Run() {
 	// handles `sendmail -bs`
 	// telnet directly to SMTP
 	if UseB && UseS {
-		var caller telnet.Caller = telnet.StandardCaller
-
-		if isSocket {
+		var caller = telnet.StandardCaller
+		switch isSocket {
+		case true:
 			if err := telnet.DialToAndCallUnix(socketAddr, caller); err != nil {
 				fmt.Println(err)
 				os.Exit(1)
 			}
-		} else {
+		default:
 			if err := telnet.DialToAndCall(SMTPAddr, caller); err != nil {
 				fmt.Println(err)
 				os.Exit(1)