咨询：Outlook插件服务器宕机时如何避免邮件无法发送

问题背景 #

我们开发的Outlook Add-in正常运行无异常，但当承载Add-in代码的服务器宕机时，用户无法发送邮件，且收到错误提示：

"Outlook can't send this message because there's a problem with the add-in [Add-in name]"

我们已尝试过相关主题的解决方案及官方策略调整，但问题仍未解决，现需实现服务器宕机时用户仍能正常发送邮件的目标。

当前Manifest配置 #

<OfficeApp xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"
           xmlns:mailappor="http://schemas.microsoft.com/office/mailappversionoverrides/1.0"
           xsi:type="MailApp">
  <Id>some guid</Id>
  <Version>1.0.0.0</Version>
  <ProviderName>Test ProviderName</ProviderName>
  <DefaultLocale>da-DK</DefaultLocale>
  <DisplayName DefaultValue="Add-in name"/>
  <Description DefaultValue="Test description"/>

  <IconUrl DefaultValue="https://localhost:3000/assets/icon-64.png"/>
  <HighResolutionIconUrl DefaultValue="https://localhost:3000/assets/icon-128.png"/>

  <SupportUrl DefaultValue="https://localhost:3000/help"/>

  <AppDomains>
    <!-- All domains you want to access must be listed here (origin) -->
    <AppDomain>https://localhost:3000/</AppDomain>
    <AppDomain>https://localhost:5001/</AppDomain>

  </AppDomains>

  <Hosts>
    <Host Name="Mailbox"/>
  </Hosts>

  <Requirements>
    <Sets>
      <Set Name="Mailbox" MinVersion="1.1"/>
    </Sets>
  </Requirements>

   <!-- Old schema. This will be overridden by the VersionOverrides below. -->
  <FormSettings>
    <Form xsi:type="ItemEdit">
      <DesktopSettings>
        <SourceLocation DefaultValue="https://localhost:3000/commands.html"/>
      </DesktopSettings>
    </Form>
  </FormSettings>
  <!-- /Old schema. -->

  <Permissions>ReadWriteMailbox</Permissions>

  <Rule xsi:type="RuleCollection" Mode="Or">
    <Rule xsi:type="ItemIs" ItemType="Message" FormType="Edit" />
  </Rule>

  <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
    <!-- On Send requires VersionOverridesV1_1 -->
    <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
      <Description resid="residAppDescription" />
      <Requirements>
        <bt:Sets DefaultMinVersion="1.3">
          <bt:Set Name="Mailbox" />
        </bt:Sets>
      </Requirements>

      <Hosts>
        <Host xsi:type="MailHost">
          <!-- Event-based activation happens in a lightweight runtime.-->
          <Runtimes>
            <!-- HTML file including reference to or inline JavaScript event handlers.
                 This is used by Outlook on the web and Outlook on the new Mac UI. -->
            <Runtime resid="WebViewRuntime.Url">
              <!-- JavaScript file containing event handlers. This is used by Outlook on Windows. -->
              <Override type="javascript" resid="JSRuntime.Url"/>
            </Runtime>
          </Runtimes>

          <DesktopFormFactor>
            <FunctionFile resid="WebViewRuntime.Url" />
            <ExtensionPoint xsi:type="Events">
              <!-- Using ItemSend allows us to open dialogs from the send event, as opposed to when using the
                  "Smart-Alert" on send handler.
                  The draw-back of this is that we cannot publish this add-in to Microsoft's AppSource.

                  The function must be on the global scope.
               -->
              <Event Type="ItemSend" FunctionExecution="synchronous" FunctionName="onMessageSendHandler" />
            </ExtensionPoint>

            <ExtensionPoint xsi:type="LaunchEvent">
              <LaunchEvents>
                <LaunchEvent Type="OnMessageCompose" FunctionName="onMessageComposeHandler"/>
                <LaunchEvent Type="OnMessageAttachmentsChanged" FunctionName="onMessageAttachmentsChangedHandler"/>
              </LaunchEvents>
              <SourceLocation resid="WebViewRuntime.Url"/>
            </ExtensionPoint>

          </DesktopFormFactor>
        </Host>
      </Hosts>
      <Resources>
        <bt:Urls>
          <!-- The JavaScript code is hosted on a secure and trusted web server. -->
          <bt:Url id="WebViewRuntime.Url" DefaultValue="https://localhost:3000/commands.html"/>
          <bt:Url id="JSRuntime.Url" DefaultValue="https://localhost:3000/commands.js" />
        </bt:Urls>
      </Resources>
    </VersionOverrides>
  </VersionOverrides>

</OfficeApp>

解决方案 #

问题根源 #

当前配置中ItemSend事件使用了同步执行模式（FunctionExecution="synchronous"），这种模式下Outlook会阻塞邮件发送流程，等待Add-in的处理结果。如果服务器宕机导致脚本无法加载或接口调用失败，Outlook会直接判定Add-in异常，阻止邮件发送。

具体修复步骤 #

1. 切换为异步事件处理（优先推荐）

将ItemSend事件的执行模式改为异步，这样Outlook不会等待Add-in处理完成，即使服务器不可用，邮件也能正常发出。

修改Manifest中的Event节点：

<Event Type="ItemSend" FunctionExecution="async" FunctionName="onMessageSendHandler" />

同时调整onMessageSendHandler函数，通过Promise处理异步逻辑，服务器不可用时直接允许发送：

async function onMessageSendHandler(event) {
  try {
    // 原有调用远程服务的逻辑
    await fetch('你的业务接口地址');
    // 服务正常时的处理逻辑
    event.completed({ allowEvent: true });
  } catch (error) {
    // 服务器不可用/调用失败时，直接放行邮件
    event.completed({ allowEvent: true });
  }
}

2. 同步模式下的超时容错（若必须保留同步）

如果因为业务需求必须使用同步模式（例如需要在发送前修改邮件内容），则在代码中添加超时逻辑，避免长时间等待服务器响应：

function onMessageSendHandler(event) {
  // 调用远程服务
  const serviceRequest = fetch('你的业务接口地址');
  // 设置5秒超时
  const timeoutPromise = new Promise(resolve => setTimeout(() => resolve(true), 5000));

  // 优先取先完成的结果
  Promise.race([serviceRequest, timeoutPromise])
    .then(() => event.completed({ allowEvent: true }))
    .catch(() => event.completed({ allowEvent: true }));
}

3. 确认Manifest的兼容性

当前Manifest中Mailbox Set的最低版本为1.3，已经支持异步ItemSend事件，无需额外调整版本要求。

关键注意事项 #

• 异步模式下无法直接修改邮件内容，若需修改需使用Smart Alert模式（但该模式无法发布到AppSource，与你当前Manifest注释中的说明一致）
• 无论同步还是异步模式，处理函数必须调用event.completed()，否则Outlook会一直等待导致发送失败
• 服务器异常时，必须明确返回allowEvent: true，确保Outlook继续执行发送流程

内容的提问来源于stack exchange，提问作者Alexander Byrka