@PostMapping("/post", produces = ["application/xml"])
fun post(@Valid request: RequestData): Mono<Response> {
    return Mono.just(request)
    ...
    ...
    ...
}

data class RequestData(

    @get:NotBlank
    @get:Email
    val email: String = "",
)

@ExceptionHandler
fun bindingExceptionHandler(e: WebExchangeBindException) = "Custom Error Message"

@Test
fun testWhenInvalidEmail() {

    // Request body
    val email = "invalidemail"
    val request = LinkedMultiValueMap<String, String>()
    request.add("email", email)

    webTestClient.post().uri("/post")
        .body(BodyInserters.fromFormData(request))
        .exchange()
        .expectStatus().isOk
}