解决方案：

1. 首先，使用AWS控制台或AWS CLI更新lambda函数的配置，将环境变量设置为以下内容：

{
  "SENDER_EMAIL_ADDRESS": "yourSenderEmailAddress@example.com",
  "SENDER_EMAIL_NAME": "Your Sender Name",
  "EMAIL_SUBJECT": "Your Email Subject",
  "EMAIL_BODY": "Your Email Body"
}

2. 然后，使用以下代码示例设置CustomEmailSender：

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import com.amazonaws.services.simpleemail.AmazonSimpleEmailService;
import com.amazonaws.services.simpleemail.AmazonSimpleEmailServiceClientBuilder;
import com.amazonaws.services.simpleemail.model.Body;
import com.amazonaws.services.simpleemail.model.Content;
import com.amazonaws.services.simpleemail.model.Destination;
import com.amazonaws.services.simpleemail.model.Message;
import com.amazonaws.services.simpleemail.model.SendEmailRequest;
import com.amazonaws.services.simpleemail.model.SendEmailResult;
import com.amazonaws.services.cognitoidp.AWSCognitoIdentityProvider;
import com.amazonaws.services.cognitoidp.AWSCognitoIdentityProviderClientBuilder;
import com.amazonaws.services.cognitoidp.model.AdminGetUserRequest;
import com.amazonaws.services.cognitoidp.model.AdminGetUserResult;
import com.amazonaws.services.cognitoidp.model.GetUserRequest;
import com.amazonaws.services.cognitoidp.model.GetUserResult;
import com.amazonaws.services.cognitoidp.model.UserNotFoundException;
import com.amazonaws.services.cognitoidp.model.UsernameExistsException;

import java.util.HashMap;
import java.util.Map;

public class CustomEmailSender implements RequestHandler, String> {

    String SENDER_EMAIL_ADDRESS = System.getenv("SENDER_EMAIL_ADDRESS");
    String SENDER_EMAIL_NAME = System.getenv("SENDER_EMAIL_NAME");
    String EMAIL_SUBJECT = System.getenv("EMAIL_SUBJECT");
    String EMAIL_BODY = System.getenv("EMAIL_BODY");

    String COGNITO_REGION = System.getenv("COGNITO_REGION");
    String USER_POOL_ID = System.getenv("USER_POOL_ID");
    String CLIENT_ID = System.getenv("CLIENT_ID");

    private AWSCognitoIdentityProvider cognitoIdentityProviderClient = null;

    private AmazonSimpleEmailService sesClient = null;

    private void init() {
        if(cognitoIdentityProviderClient == null) {
            cognitoIdentityProviderClient = AWSCognitoIdentityProviderClientBuilder.standard()
                    .withRegion(COGNITO_REGION)
                    .build();
        }
        if(sesClient == null) {
            sesClient = AmazonSimpleEmailServiceClientBuilder.standard()
                    .withRegion(System.getenv("AWS_REGION"))
                    .build();
        }
    }

    @Override
     public String handleRequest(Map event, Context context) {
        try {
            init();
            String email = null;
            Object userEmailObj = event.get("userEmail");
            if(userEmailObj != null) {
                email = userEmailObj.toString();
            }
            String userId = null;
            Object cognitoUserObj = event.get("cognitoUser");
            if(cognitoUserObj != null) {
                Map cognitoUser = (Map)cognitoUserObj;
                userId = cognitoUser.get("username").toString();
                AdminGetUserRequest adminGetUserRequest = new AdminGetUserRequest()
                        .withUserPoolId(USER_POOL_ID)
                        .withUsername(userId);
                AdminGetUserResult adminGetUserResult = cognitoIdentityProviderClient.adminGetUser(adminGetUserRequest);
                Map userAttributes = new HashMap<>();
                for(com.amazonaws.services.cognitoidp.model.AttributeType attribute : adminGetUserResult.getUserAttributes()) {
                    userAttributes.put(attribute.getName(), attribute