You wish to configure an SMTP connector for Letsignit in Google Workspace or O365, so that the Letsignit signature is applied regardless of which device they send the email from.
For information on how to configure the SMTP Connector for O365, please refer to this article.
If you run into an error stating "No relay was found. It is not possible to proceed with the configuration. Please verify your domain name" during the configuration, please refer to the attached "LSI O365" document for an alternative configuration path.
There is also a possibility that the tenant will have other connectors already configured. Some examples could include Proofpoint, Protected Trust, etc.
It is important to note that for the mail flow to work properly, the Letsignit rules that are created along with the connector will need to be set as the top priority. In the exchange admin center this would be done under mail flow > rules and then selecting the rule and moving it up in the list using the up arrow as seen here:
The rule priority would then be updated as seen here:
Google Workspace Connector
For information on how to configure the SMTP Connector for Google workspace, please refer to the attached "LSI Google" document.