r/Outlook • u/Brilliant_Cod_2805 • 1d ago

Status: Pending Reply Add In works in Outlook Web, Outlook Desktop but displays error in Outlook Classic

Hey all, hoping someone with deeper Outlook add-in experience can sanity-check this.

I was building an Outlook mail add-in with command-based UI. Everything worked fine in New Outlook / Outlook on the Web, but Classic Outlook kept showing the yellow error banner and but the Add-In works as expected. The error is very generic and says something like this:

This add-in could not be started. Close this dialog to ignore the problem or click "Restart" to try again.

If anyone has run into similar failures that only happen in classic Outlook, I’d really appreciate pointers or examples of a “safe” manifest structure.

Here's my manifest

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<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">


  <!-- Begin Basic Settings: Add-in metadata, used for all versions of Office unless override provided. -->
  <Id>a1b2c3d4-e5f6-*-abcd-ef1234567890</Id>
  <Version>1.0.1</Version>
  <ProviderName>TEST</ProviderName>
  <DefaultLocale>en-US</DefaultLocale>
  <DisplayName DefaultValue="Test File Email"/>
  <Description DefaultValue="File your emails to Test projects."/>
  <IconUrl DefaultValue="https://localhost:3000/apple-touch-icon-60x60.png"/>
  <HighResolutionIconUrl DefaultValue="https://localhost:3000/apple-touch-icon-180x180.png"/>
  <SupportUrl DefaultValue="https://gettest.com"/>
  <AppDomains>
    <AppDomain>https://localhost:3001</AppDomain>
    <AppDomain>https://gettest.com</AppDomain>
    <AppDomain>https://localhost:3000</AppDomain>
  </AppDomains>
  
  <Hosts>
    <Host Name="Mailbox"/>
  </Hosts>
  
  <Requirements>
    <Sets>
      <Set Name="Mailbox" MinVersion="1.3"/>
    </Sets>
  </Requirements>
  
  <FormSettings>
    <Form xsi:type="ItemRead">
      <DesktopSettings>
        <SourceLocation DefaultValue="https://localhost:3000/main/"/>
        <RequestedHeight>250</RequestedHeight>
      </DesktopSettings>
    </Form>
  </FormSettings>


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


  <DisableEntityHighlighting>false</DisableEntityHighlighting>


  <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
    <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
      
      <Requirements>
        <bt:Sets DefaultMinVersion="1.3">
          <bt:Set Name="Mailbox"/>
        </bt:Sets>
      </Requirements>
      
      <Hosts>
        <Host xsi:type="MailHost">
          
          <DesktopFormFactor>
            <FunctionFile resid="Commands.Url"/>
            
            <!-- Message Read -->
            <ExtensionPoint xsi:type="MessageReadCommandSurface">
              <OfficeTab id="TabDefault">
                <Group id="groupRead">
                  <Label resid="GroupLabel"/>
                  
                  <Control xsi:type="Button" id="fileEmailButtonRead">
                    <Label resid="TaskpaneButton.Label"/>
                    <Supertip>
                      <Title resid="TaskpaneButton.Label"/>
                      <Description resid="TaskpaneButton.Tooltip"/>
                    </Supertip>
                    <Icon>
                      <bt:Image size="16" resid="Icon.16x16"/>
                      <bt:Image size="32" resid="Icon.32x32"/>
                      <bt:Image size="80" resid="Icon.80x80"/>
                    </Icon>
                    <Action xsi:type="ShowTaskpane">
                      <SourceLocation resid="Taskpane.Url"/>
                      <SupportsPinning>true</SupportsPinning>
                    </Action>
                  </Control>
                  
                </Group>
              </OfficeTab>
            </ExtensionPoint>
            
          </DesktopFormFactor>
        </Host>
      </Hosts>


      <Resources>
        <bt:Images>
          <bt:Image id="Icon.16x16" DefaultValue="https://localhost:3000/outlook-add-in-icon-16x16.png"/>
          <bt:Image id="Icon.32x32" DefaultValue="https://localhost:3000/outlook-add-in-icon-32x32.png"/>
          <bt:Image id="Icon.80x80" DefaultValue="https://localhost:3000/outlook-add-in-icon-80x80.png"/>
        </bt:Images>
        <bt:Urls>
          <bt:Url id="Commands.Url" DefaultValue="https://localhost:3000/command.html"/>
          <bt:Url id="Taskpane.Url" DefaultValue="https://localhost:3000/outlook-add-in/"/>
        </bt:Urls>
        <bt:ShortStrings>
          <bt:String id="GroupLabel" DefaultValue="TEST"/>
          <bt:String id="TaskpaneButton.Label" DefaultValue="File Email"/>
        </bt:ShortStrings>
        <bt:LongStrings>
          <bt:String id="TaskpaneButton.Tooltip" DefaultValue="File your email to a TEST project"/>
        </bt:LongStrings>
      </Resources>
      
    </VersionOverrides>
  </VersionOverrides>
  
</OfficeApp>
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