javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at com.sun.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:2000)

Caused by: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

<bean id="mailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl">
    <property name="host" value="${mail.host}"/>
    <property name="port" value="${mail.port}"/>
    <property name="username" value="${mail.username}"/>
    <property name="password" value="${mail.password}"/>
    <property name="javaMailProperties">
        <props>
            <prop key="mail.transport.protocol">${mail.transport.protocol}</prop>
            <prop key="mail.smtp.auth">${mail.smtp.auth}</prop>
            <prop key="mail.smtp.starttls.enable">${mail.smtp.starttls.enable}</prop>
            <prop key="mail.smtp.ssl.trust">${mail.smtp.ssl.trust}</prop>
        </props>
    </property>
</bean>

mail.host=smtp.gmail.com
mail.port=587
mail.username=user@gmail.com
mail.password=password
mail.transport.protocol=smtp
mail.smtp.auth=true
mail.smtp.starttls.enable=true
mail.smtp.ssl.trust=smtp.gmail.com

Email email = new SimpleEmail();
email.setHostName("smtp.googlemail.com");
email.setSmtpPort(465);
email.setAuthenticator(new DefaultAuthenticator("username", "password"));
email.setSSLOnConnect(true);
email.setFrom("user@gmail.com");
email.setSubject("TestMail");
email.setMsg("This is a test mail ... :-)");
email.addTo("foo@bar.com");
email.send();

Properties props = new Properties();
        MailSSLSocketFactory socketFactory = new MailSSLSocketFactory();
        socketFactory.setTrustAllHosts(true);
        props.put("mail.imaps.ssl.socketFactory", socketFactory);

<dependency>
  <groupId>javax.mail</groupId>
  <artifactId>mail</artifactId>
  <version>1.5.0-b01</version>
</dependency>