Creating an Internal Signature
Create a new rule in the Rules section > In the BUILDER tab, select 'Internal Emails Only' and click SAVE.
When creating a signature, select Rule under 'Who is this signature for?' internal rule that you created in Step 2.
Creating an External Signature
To create an external signature, follow steps 1 and 2 but instead of choosing 'Internal Emails Only' select 'External Emails Only.