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?