Please check the following things.DeploymentA) Ribbon ButtonFirst, check if the deployment is working. In Outlook for Windows, you should see the 'Organizational Signature' icon (red circle below) in the ribbon on a newly created e-mail. You can skip this section and continue in the permissions section if you see the icon.B) Unified deploymentPlease make sure you have deployed the Mail Signature using Unified Deployment in Microsoft 365 Admin Center to users who are not getting the mail signature automatically. Be aware that it can take a few hours for the app to appear in Outlook.C) Outlook OnlineCheck to see if you are getting the Mail Signature in Outlook Online.D) Mail accountCheck that you are using the mail account that has the Mail Signature Add-in deployed. Be aware that if you use multiple e-mail accounts, the ribbon will only show the 'Organizational Signatur' button if you use the Mail account that has the Add-in deployed. Tenant SettingsE) Modern authentication enabledCheck if modern authentication is enabled for your tenant. In the Microsoft 365 admin center, go to Settings > Org Settings > Modern Authentication. In the Modern authentication flyout that appears, click to enable or disable.Enable or disable modern authentication for Outlook in Exchange Online | Microsoft DocsPermissionsF) Pre-ConsentEnsure you have pre-consented all Mail Signature permissions using the officeatwork Admin Center. Be sure to re-launch Outlook after granting the permissions.G) Re-DeploymentSome customers have seen error messages like 'Could not access Mail Signature' on one or multiple devices. Please check your network connection...'. We have seen customers fix this by deleting all granted permissions via Enterprise Applications in Azure and then re-deploying the add-ins. Note that there are two permission, one set during deployment via centralized deployment in Microsoft 365 and one set via the officeatwork admin center:mail-signatureofficeatwork Mail SignatureHere are the steps customers have taken to fix this:Remove both Enterprise Application entries for Mail SignatureRemove the Mail Signature for Outlook deployment(wait until it got removed from Outlook - this can take up to 6 hours)Deploy the Mail Signature for Outlook via Microsoft 365 centralized deploymentPre-consent all the permissions via the officeatwork Admin CenterCheck to see if the Mail Signature now inserts automatically(note that you might have to wait up to 6 hours for the deployment to complete, see message provided during the deployment)Outlook VersionH) Insert manuallyCheck if you can insert the mail signature manually using the 'Organizational Signature' button in the ribbon.I) Outlook OnlineCheck if the signature is automatically inserted in Outlook Online. If yes, check your version of Outlook to see if it already supports the automatic insertion of mail signatures. Learn more here: Mail Signature - compatibility overview.Network settingsJ) .well-known filesCheck to see if you are experiencing any issues loading the well-known files. https://cdn.officeatwork365.com/.well-known/microsoft-officeaddins-allowed.jsonhttps://cdn.officeatwork365.com/mail-signature/runtime.jsUse Fiddler to check the network traffic when creating a new e-mail message.