This file merges mailgun validation functionality with jQuery Validation Plugin's surprisingly poor async validation customizability.

mailgun_validate.js

/**
 * Created by zevisert on 2017-03-27.
 * This file merges mailgun validation functionality with jQueryValidationPlugin's surprisingly
 * poor async validation customizability. Much of this code has been borrowed from
 *   * jQuery Validation remote method
 *      - https://github.com/jquery-validation/jquery-validation/blob/master/src/core.js#L1477
 *   * Mailgun validator jquery plugin
 *      - https://github.com/mailgun/validator-demo/blob/master/mailgun_validator.js#L30
 *
 *
 * Mailgun Address Validation Plugin with jQuery Validation plugin support
 *
 * Attaching to a form:
 *
 *  | $('form_selector').mailgun_validator({
 *  |     api_key: 'api-key',
 *  |     in_progress: in_progress_callback, // called when request is made to validator
 *  |     success: success_callback,         // called when validator has returned
 *  |     error: validation_error,           // called when an error reaching the validator has occured
 *  | });
 *
 * Usage in jquery validation:
 *
 *  | $('form_selector').validate({
 *  |     rules: {
 *  |         email_address: {
 *  |             required: true,
 *  |             mailgun: '<public-api-key>'
 *  |         }
 *  |     },
 *  |     messages: {
 *  |         email_address: {
 *  |             required: "Please enter your email address.",
 *  |             mailgun: "Please enter a valid email address."
 *  |         }
 *  |     }
 *  | });
 *
 *  > then (in form_validation.js):
 *
 *  | jQuery.validator.addMethod('mailgun', function (value, element, api_key) {
 *  |     var preflight_result = jQuery.run_mailgun_validator(value, {
 *  |         api_key: api_key,
 *  |         success: success_callback,
 *  |         error: error_callback,
 *  |         in_progress: in_progress_callback,
 *  |         validator: this
 *  |     }, element);
 *  |
 *  |     return typeof preflight_result === "undefined" ? "pending" : preflight_result;
 *  | });
 *
 * Sample JSON in success callback:
 *
 *  | {
 *  |     "is_valid": true,
 *  |     "parts": {
 *  |         "local_part": "john.smith@example.com",
 *  |         "domain": "example.com",
 *  |         "display_name": ""
 *  |     },
 *  |     "address": "john.smith@example.com",
 *  |     "did_you_mean": null
 *  | }
 *
 * More API details: https://api.mailgun.net/v2/address
 * https://jqueryvalidation.org/documentation/
 **/

(function( $ ) {
	$.fn.mailgun_validator = function(options) {
		return this.each(function() {
			var thisElement = $(this);
			thisElement.focusout(function(e) {
				//Trim string and autocorrect whitespace issues
				var elementValue = thisElement.val();
				elementValue = $.trim(elementValue);
				thisElement.val(elementValue);

				//Attach event to options
				options.e = e;

				$.run_mailgun_validator(elementValue, options, thisElement);
			});
		});
	};

	$.extend({run_mailgun_validator: function(address_text, options, element) {
		const method = 'mailgun';
		// require api key
		if (options && typeof options.api_key === 'undefined') {
			if (console) console.warn('Please pass in api_key to mailgun_validator.');
		}

		// don't run validator without input
		if (!address_text) {
			if (options.validator) {
				options.validator.stopRequest(element, false);
				return true;
			} else {
				return;
			}
		}

		// don't run duplicate calls
		var previous;
		if (options.validator){
			// Use jQuery Validator duplicate logic checks if available
			previous = options.validator.previousValue( element, method );

			if ( !options.validator.settings.messages[ element.name ] ) {
				options.validator.settings.messages[ element.name ] = {};
			}
			previous.originalMessage = previous.originalMessage || options.validator.settings.messages[ element.name ][ method ];
			options.validator.settings.messages[ element.name ][ method ] = previous.message;

			var optionDataString = $.param( { address: address_text, api_key: options.api_key } );
			if ( previous.old === optionDataString ) {
				return previous.valid;
			}
			previous.old = optionDataString;

			options.validator.startRequest( element );

			if (!options.validator.resetInternals){
				options.validator.resetInternals = function () {
					options.validator.successList = [];
					options.validator.errorList = [];
					options.validator.errorMap = {};
					options.validator.toShow = $( [] );
					options.validator.toHide = $( [] );
				}
			}
		} else if (element.mailgunLastSuccessReturn) {
			// else use mailgun duplicate checks
			if (address_text == element.mailgunLastSuccessReturn.address) {
				if (options && options.success) {
					options.success(element.mailgunLastSuccessReturn, options.e);
				}
				if (options.validator) {
					options.validator.stopRequest(element, element.mailgunLastSuccessReturn.is_valid);
					return element.mailgunLastSuccessReturn.is_valid;
				} else {
					return;
				}
			}
		}

		//Abort existing AJAX Request to prevent flooding
		if (element.mailgunRequest) {
			if (options.validator){
				previous.old = null;
				options.validator.stopRequest(element, previous.valid);
			}
			element.mailgunRequest.abort();
			element.mailgunRequest = null;
			console.info("Aborted an inflight AJAX request - expect ReferenceErrors");
		}

		// validator is now in progress
		if (options && options.in_progress) {
			options.in_progress(options.e);
		}

		// length and @ syntax check
		var error_message = false;
		if (address_text.length > 512) {
			error_message = 'Email address exceeds maxiumum allowable length of 512.';
		} else if (1 !== address_text.split('@').length-1) {
            error_message = 'Email address must contain only one @.';
        }

		if (error_message) {
			if (options && options.error) {
				options.error(error_message, options.e);
			} else {
				if (console) console.log(error_message);
			}
			if (options.validator) {
				options.validator.stopRequest(element, false);
				return false;
			} else {
				return;
			}
		}

		// Set a timeout incase of some kind of internal server error
		var timeoutID = setTimeout(function() {
			error_message = 'Timeout occurred, unable to validate address.';
			if (!options.success) {
				//Abort existing AJAX Request for a true timeout
				if(element.mailgunRequest) {
					element.mailgunRequest.abort();
					element.mailgunRequest = null;
					if (options.validator) {
						options.validator.stopRequest(element, previous.valid);
						return previous.valid;
					}
				}

				if (options && options.error) {
					options.error(error_message, options.e);
				} else {
					if (console) console.log(error_message);
				}
			}
		}, 3000); // 3 seconds

		// make ajax call to get validation results
		element.mailgunRequest = $.ajax({
			type: "GET",
			url: 'https://api.mailgun.net/v2/address/validate?callback=?',
			data: { address: address_text, api_key: options.api_key },
			dataType: "jsonp",
			crossDomain: true,
			success: function(data, status_text, request) {
				clearTimeout(timeoutID);

				element.mailgunLastSuccessReturn = data;

				if (options.validator) {
					var valid = data.is_valid === true || data.is_valid === "true";

					options.validator.settings.messages[element.name][method] = previous.originalMessage;
					var errors = {};
					var message = null;
					if (valid) {
						var submitted = options.validator.formSubmitted;
						options.validator.resetInternals();
						options.validator.toHide = options.validator.errorsFor(element);
						options.validator.formSubmitted = submitted;
						options.validator.successList.push(element);
						options.validator.invalid[element.name] = false;

						message = options.success && options.success(data, options.e) || message;
						if (message) {
							errors[element.name] = previous.message = message;
							options.validator.showErrors(errors);
						} else {
							options.validator.showErrors();
						}
					} else {
						message = options.success && options.success(data, options.e)
							|| options.validator.defaultMessage( element, { method: method, parameters: address_text } );
						errors[element.name] = previous.message = message;
						options.validator.invalid[element.name] = true;
						options.validator.showErrors(errors);
					}

					previous.valid = valid;
					options.validator.stopRequest(element, valid);
				}
				else if (options && options.success) {
					options.success(data, options.e);
				}

			},
			error: function(request, status_text, error) {
				clearTimeout(timeoutID);
				var error_message = 'Error occurred, unable to validate address.';
				var errors = {};

				if (options && options.error) {
					error_message = options.error(error_message, error) || error_message;
					errors[element.name] = error_message;
					if (options.validator) {
						previous.message = error_message;
					}
					options.validator.invalid[element.name] = true;
					options.validator.showErrors(errors);
				}
				else {
					if (console) console.log(error_message);
				}
			},
			complete: function(request, status_text){
				element.mailgunRequest = null;
			}
		});
	}
	});
})( jQuery );