解决spring boot JavaMailSender部分收件人错误导致发送失败的问题

　　使用spring boot通常使用spring-boot-starter-mail进行邮件的发送。当进行邮件群发的话，如果一个收件人的地址错误，会导致所有邮件都发送失败。因此我们需要在邮件发送失败的时候把错误的收件人移除，重新发送。

　　当邮件发送失败的时候会抛出MailSendException，异常信息中包含错误的收件人信息。

　　主要代码如下：

private void sendMail(List<String> mailList, MimeMessageHelper message){
    try{
        this.mailSender.send(message.getMimeMessage());
    }catch (MailSendException e){
        Set<String> tmpInvalidMails = getInvalidAddress(e);
        // 非无效收件人导致的异常，暂不处理
        if (tmpInvalidMails.isEmpty()){
            logger.error(e.getMessage());
            return;
        }
        mailList.removeAll(tmpInvalidMails);
        if(mailList.isEmpty()){
            logger.error("邮件发送失败，无收件人" + e.getMessage());
            return;
        }
        message.setTo(mailList.toArray(new String[0]));
        sendMail(mailList, message)
    }
}

　　捕获邮件发送失败的异常，首先判断是否是收件人无效导致的异常。从异常中解析无效收件人，收件人例表中移除无效的收件人，重新发送邮件。

　　如何从从异常中获取无效收件人，首先看下JavaMailSenderImpl这个类的doSend方法

protected void doSend(MimeMessage[] mimeMessages, @Nullable Object[] originalMessages) throws MailException {
    Map<Object, Exception> failedMessages = new LinkedHashMap<>();
    Transport transport = null;
    try {
        for (int i = 0; i < mimeMessages.length; i++) {
            ...
            // Send message via current transport...
            MimeMessage mimeMessage = mimeMessages[i];
            try {
                ...
            }
            catch (Exception ex) {
                Object original = (originalMessages != null ? originalMessages[i] : mimeMessage);
                failedMessages.put(original, ex);
            }
        }
    }
    finally {
        try {
            if (transport != null) {
                transport.close();
            }
        }
        catch (Exception ex) {
            if (!failedMessages.isEmpty()) {
                throw new MailSendException("Failed to close server connection after message failures", ex,
                        failedMessages);
            }
            else {
                throw new MailSendException("Failed to close server connection after message sending", ex);
            }
        }
    }

    if (!failedMessages.isEmpty()) {
        throw new MailSendException(failedMessages);
    }
}

　　当邮件发送过程中遇到异常会保存到failedMessages中，我们需要从中解析收件人无效导致的异常。

　　接着继续看源码SMTPTransport的rcptTo方法，会去校验每个收件人，通过向服务器发送RCPT TO:<地址>，根据响应码来判断收件人是否有效。

protected void rcptTo() throws MessagingException {
    List<InternetAddress> valid = new ArrayList();
    List<InternetAddress> validUnsent = new ArrayList();
    List<InternetAddress> invalid = new ArrayList();
    ...
    int k;
    for(k = 0; k < this.addresses.length; ++k) {
        sfex = null;
        InternetAddress ia = (InternetAddress)this.addresses[k];
        String cmd = "RCPT TO:" + this.normalizeAddress(ia.getAddress());
        if (dsn) {
            cmd = cmd + " NOTIFY=" + notify;
        }

        this.sendCommand(cmd);
        int retCode = this.readServerResponse();
        switch(retCode) {
        case 250:
        case 251:
            valid.add(ia);
            ...
            break;
        case 450:
        case 451:
        case 452:
        case 552:
            ...
            validUnsent.add(ia);
            ...
            break;
        case 501:
        case 503:
        case 550:
        case 551:
        case 553:
            ...
            invalid.add(ia);
            ...
            break;
        default:
            if (retCode >= 400 && retCode <= 499) {
                validUnsent.add(ia);
            } else {
                ...
                invalid.add(ia);
            }
            ...
        }
    }

    if (sendPartial && valid.size() == 0) {
        sendFailed = true;
    }

    int lrc;
    if (sendFailed) {
        this.invalidAddr = new Address[invalid.size()];
        invalid.toArray(this.invalidAddr);
        this.validUnsentAddr = new Address[valid.size() + validUnsent.size()];
        k = 0;

        for(lrc = 0; lrc < valid.size(); ++lrc) {
            this.validUnsentAddr[k++] = (Address)valid.get(lrc);
        }

        for(lrc = 0; lrc < validUnsent.size(); ++lrc) {
            this.validUnsentAddr[k++] = (Address)validUnsent.get(lrc);
        }
    } 
    ...

    if (sendFailed) {
        this.logger.fine("Sending failed because of invalid destination addresses");
        ...
        throw new SendFailedException("Invalid Addresses", (Exception)mex, this.validSentAddr, this.validUnsentAddr, this.invalidAddr);
    }
}

　　当收件人无效发送失败会抛出SendFailedException异常，异常中包含收件人是否有效的信息。

　　因此我们只要从failedMessages查找是否含有SendFailedException，然后从SendFailedException直接得到无效的收件人信息。代码如下：

    private Set<String> getInvalidAddress(MailSendException e){
        Set<String> mails = new HashSet<>();
        for(Exception exception: e.getFailedMessages().values()){
            if(exception instanceof SendFailedException){
                for(Address address: ((SendFailedException) exception).getInvalidAddresses()){
                    mails.add(address.toString());
                }
            }
        }
        return mails;
    }