If you are a Heroku user using the SendGrid Add-on, and would like to access the SendGrid web interface, you can find your SendGrid credentials by issuing the following command:
heroku config --long
You can also go the other direction and configure heroku to use an existing SendGrid account and password. Just run the following commands from your shell:
heroku config:add SENDGRID_USERNAME=existing_username
heroku config:add SENDGRID_PASSWORD=existing_password
Make sure that your SENDGRID_DOMAIN config var matches the website field in your SendGrid profile.