App Engine不发送邮件

Question

我有以下用于发送邮件的代码，但是在我尝试的某些日子里它没有工作。我已经按照所有可能的描述来完成这项工作，但仍然没有办法。我使用javax邮件服务发送邮件，我也更新了所有依赖项，但日志将显示已调用邮件功能，但我不会收到任何邮件。

import com.dotdex.squattn.models.UserMail;
import com.dotdex.squattn.util.Utility;
import com.firebase.client.Firebase;
import com.google.api.server.spi.config.Api;
import com.google.api.server.spi.config.ApiMethod;
import com.google.api.server.spi.config.ApiNamespace;

import java.io.UnsupportedEncodingException;
import java.util.Properties;
import java.util.logging.Logger;

import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.AddressException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;


/**
 * An endpoint class we are exposing
 */
@Api(
        name = "userMailApi",
        version = "v1",
        resource = "userMail",
        namespace = @ApiNamespace(
                ownerDomain = "squattn.dotdex.com",
                ownerName = "squattn.dotdex.com",
                packagePath = ""
        )
)
public class UserMailEndpoint {

    private static final Logger logger = Logger.getLogger(UserMailEndpoint.class.getName());

    /**
     * This inserts a new <code>UserMail</code> object.
     *
     * @param userMail The object to be added.
     * @return The object to be added.
     */
    @ApiMethod(name = "sendUserConfirmationMail")
    public UserMail sendUserConfirmationMail(UserMail userMail) {
        logger.info("Calling insertUserMail method");
        if(sendMail(userMail))
        {
            userMail.setMailSent(true);
        }

        return userMail;
    }

    private boolean sendMail(UserMail userMail) {
        //start sending the mail
        Properties props = new Properties();
        Session session = Session.getDefaultInstance(props, null);

        String link = "<br><br><a href='" +Utility.APP_URL + "/verifyEmail?uid=" + userMail.getUserId() + "' style='color: #ffffff; text-decoration: none; margin: 0px; text-align: center; vertical-align: baseline; border: 4px solid #6fb3e0; padding: 4px 9px; font-size: 15px; line-height: 21px; background-color: #6fb3e0;'>&nbsp; Confirm &nbsp;</a>";

        try {

            Multipart mp = new MimeMultipart();
            MimeBodyPart htmlPart = new MimeBodyPart();
            htmlPart.setContent(Utility.TOP_HTML + link + Utility.BOTTOM_HTML, "text/html");
            mp.addBodyPart(htmlPart);

            Message msg = new MimeMessage(session);
            msg.setFrom(new InternetAddress("noreply@parkpal-1278.appspotmail.com","ParkPal App"));
            msg.addRecipient(Message.RecipientType.TO,
                    new InternetAddress("jerryhanksokafor@gmail.com", userMail.getUserName()));
            msg.setSubject("Verify Your Email.");
            msg.setContent(mp);
            Transport.send(msg);
            new Firebase(Utility.APP_URL).child(Utility.USER_STRING).child(userMail.getUserId()).child("verifyMailSent").setValue(true);
            return true;
        } catch (AddressException e) {
            // ...
            e.printStackTrace();
            return false;
        } catch (MessagingException e) {
            // ...
            e.printStackTrace();
            return false;
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
            return false;
        }
    }
}