What are working Monitored Inbox Settings for Mautic to Fetch Emails via IMAP?

Scenario: I set up a fresh Linux / Plesk VPS using Postfix / Dovecot, and of course Mautic. I want to use my own MTA / SMTP / IMAP services that are on the same server as where Mautic is installed. Not an outside service.

Mautic can send email. What it can’t do is Fetch email via IMAP.

I have tried essentially every conceivable combination of settings under ‘Monitored Inbox Settings’: localhost / 127.0.0.1 / actual IMAP URL; all possible ports (143, 993), all encryption options. No matter what, it will invariably fail with: Error communicating with the IMAP server: TLS/SSL failure for 127.0.0.1: SSL negotiation failed PHP Notice: Unknown: TLS/SSL failure for 127.0.0.1: SSL negotiation failed (errflg=2) in Unknown on line 0

or similar depending on the settings I am using at the moment. Obviously here I was using 127.0.0.1 for the IMAP URL. This error occurs even when Encryption is set to None.

I have not been able to find any useful information on this error, which is found in the Dovecot logs and is also the error logged by the CRON job mautic:email:fetch.

Obviously, the IMAP services work perfectly well from anywhere excepting Mautic.

Additional: On the rare occasions that clicking the test button results in ‘Success!’, the CRON job says ‘No mailboxes are configured’ even though there of course is a mailbox and password that it is using. And still no email is fetched.