Can you create that folder again and let it sit and see what happens.
When in round cube make sure delivery status notification is on and try sending an email, to see if a status message comes through.
Upon further checking, it is a SSL problem since your SSL is only covering your domain itself and no subdomains. I would either recommend changing the set mail server to just be the domain itself or the server hostname.
Free subdomains are not compatible with Cloudflare.
The rate limit can not be bypassed. All you can do is try to change ACME Provider setting between let’s encrypt and ZeroSSL or:
Use service like sslforfree to create a certificate and download the zip file it provides and extract it.
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