Try looking at entries in /var/log/mail.log for the message you are waiting for, as it is likely being delayed by postgrey. This can really take a long time if the sender keeps re-sending using an MTA on a different IP address. If this is the case, I created a guide with the intent of showing how to configure the postgrey whitelist:
It looks like you have an MX record that is an IP address and it should be a hostname. It is somewhat possible this will resolve inboxing issues. Since you are using an external DNS server for your domain, you will need to double-check that everything matches what MiaB is expecting for mail-related records. Usually the MiaB dashboard will report these kinds of things as errors.
For providers like Google that are habitually declaring your mail as spam, is there anything in the headers that Google thinks is not correct? Do they leave any messages for you in the mail log transactions?