I just reviewed your DNS and Plesk settings. Your mailboxes are set up correctly and Roundcube works on the email subdomain. For DNS, I don't see the TXT records for SPF and DKIM, but it's possible I can't see them due to Cloudflare. Those would only affect sending email anyway, not receiving it.
Silly question, but with the delivery issue, are you talking about the Plesk mailbox that's forwarded to gmail? Gmail will not accept forwarded mail that originally came from itself, so if you send a message from Gmail to a Plesk mailbox that has been forwarded back to gmail, the message will simply disappear. This is a limitation imposed by Google.
Also, for forwarded mailboxes, messages aren't kept in Plesk (and so won't show in Roundcube) unless you clear the "Don't deliver copies to the plesk mailbox" option...
Finally, keep in mind that forwarded emails count towards your 50/day limit, so getting a sudden wave of unwanted incoming mail could result in your email service being removed.