SendGrid X-SMTPAPI Headers for Node.js

The `smtpapi` library is an official Twilio SendGrid Node.js module designed to programmatically construct X-SMTPAPI headers. These headers enable advanced email features such as scheduled sends, substitutions, categories, unique arguments, and recipient lists for emails sent via SendGrid's SMTP API or older V2 Mail Send API. The current stable version is 1.4.7. The package has a maintenance-focused release cadence, with recent updates primarily consisting of chore and documentation improvements rather than new features. While functional, it's crucial to note that newer SendGrid V3 Mail Send API integrations typically utilize `custom_args` directly within the API request body, which differs from the X-SMTPAPI header approach this library facilitates. This library serves as a dedicated tool for those using the SMTP or V2 API methods that rely on the X-SMTPAPI header format.