Signatures that are attached via mail clients will not be included in those sent via the Nylas API, the only way email signatures can be included is via the provider - SMTP level.
You will need to discuss the adoption of this with your customers Mail administrator.
To troubleshoot why emails do not have signatures:
- Have an email with and without the signature
- Open them in a text editor and compare
- Typically applications such as Exclaimer require a placeholder
- If required ensure the application built using Nylas has the option to insert this placeholder.