If you have already read this article:
Should I use the Add-in or the Agent?
Which solution to choose when connecting to Signitic?
Then you are probably in the right place to download the Signitic agent. Here are the 3 articles that can help you on this topic:
Install the agent for Windows Outlook
Installation guide for Outlook on Windows
Install the agent for Outlook MacOS
Installation guide for Outlook on MacOS
Install the agent for Apple Mail
Installation guide for Apple Mail
Do not hesitate to contact support if you need help on this topic!