Can you make an appointment to live chat with me? It will be fast to debug. It is taking more time than it should be.
If yes, give me time range in GMT when you can.

Let me know.

Ok, Monday 9am GMT.
Please access https://www.shamimsplugins.com/live-chat/ to chat

I believe sendgrid is to send email, not receiving.
You can ask them if they support pop3.

There are 2 features (Email Piping and POP3) to do same one thing (Receive email). Your hosting and email service should support at least one of them to make it work.
https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/email-piping/
https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/pop3-feature/

Did you ask wpengine why it fails when we use imap_open? even without an error log? Also no code is processing after that call?

Please go to Front End PM PRO > Settings > Emails > Email Piping/POP3 then set Enable as “None” then save changes.
For sending email you can use any one from https://wpengine.com/support/using-3rd-party-email-provider-send-mail-wordpress/ (eg. WP Mail SMTP) and this plugin will automatically use that plugin to send email. You can setup for gsuit/sendgrid there.