The Legacy SendSafely for Outlook plug-in is a VSTO app, which is an older method of integrating with Microsoft Outlook and requires users to install the plug-in locally on their desktop. This legacy plug-in is not compatible with Outlook on the Web and can only be installed on Windows desktops.
In contrast, the new SendSafely for Outlook add-in is built using a newer add-in framework for Outlook that works across all newer versions of Outlook on Windows and Mac as well as with the Outlook Web App (Microsoft 365 and Outlook.com). The Outlook add-in can be installed directly from Microsoft AppSource.
We will continue to support the legacy plug-in for the near future, but recommend that users begin to transition to the new add-in when possible. We will eventually be ending support for the legacy plug-in once the majority of our users transition to the newer add-in.