调用内部邮件服务器发送邮件，unable to find valid certification path to requested target

调用内部邮件服务器发送邮件时出现"unable to find valid certification path to requested target"的错误是因为本地没有目标服务器的证书。这个问题可以通过以下两种方法来解决：

方法一是在运行Java环境中安装目标服务器的证书。你可以使用keytool命令生成目标服务器证书的cer格式文件，并将其添加到Java证书库中。

方法二是在建立连接时跳过证书验证。你可以使用Apache HttpClient库来创建一个支持跳过证书验证的httpClient对象。

代码示例：

import org.apache.http.client.HttpClient;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.conn.ssl.NoopHostnameVerifier;
import org.apache.http.conn.ssl.SSLContextBuilder;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;

import javax.net.ssl.SSLContext;
import java.security.KeyManagementException;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

public class MailSender {
    public static void main(String[] args) {
        CloseableHttpClient httpClient = getSslHttpClient();
        // 使用httpClient发送邮件的代码...
    }

    public static CloseableHttpClient getSslHttpClient() {
        SSLContext sslContext = null;
        try {
            sslContext = SSLContextBuilder.create()
                    .loadTrustMaterial((x509Certificates, s) -> true)
                    .build();
        } catch (NoSuchAlgorithmException | KeyManagementException | KeyStoreException e) {
            e.printStackTrace();
        }

        return HttpClients.custom()
                .setSSLContext(sslContext)
                .setSSLHostnameVerifier(new NoopHostnameVerifier())
                .build();
    }
}