This guide will show you how to configure your SMTP profile to support using an API key with Sengrid.
Navigate to Admin → Plugins → SMTP Maintenance
Click the edit button associated with your Sengrid SMTP Profile
Modify the form fields like so
Host Name: smtp.sendgrid.net
From Email: the address associated with this SMTP Profile
password: API key from Sengrid]
Be sure to specify the appropriate port number and other server settings such as “Use SSL”, “Use Relay”, etc. based on your SMTP provider.