test emails not sending, no emails sending

Installed the Sendgrid plugin as per the documentation. Inserted the generated API key, sent tester emails using the WordPress plugin backend, these were confirmed to have been sent – confirmed by the SendGrid account. None were received, to various email addresses on various domains and nothing of the forms we are using on the site (Ninja) are being sent now either. We looked into Sendgrid to make sure all are being sent via the forms as it was hit and miss… now however nothing is coming through.