Thank you for answering that. Unfortunately, when sending a test email via your community, it says that it is successfully sent but I do not receive it when sending to Gmail addresses. I received it fine sending to my own personal email that is not as strict as Gmail but Gmail, it was never received, not even in spam.
The user you provided there is using Gmail as their email address. You can test this by going to ACP -> System -> Email Settings -> Test Settings (top right).
It could be that Gmail has blocked your server for emails, you're not passing required items in your DNS for "allowing" your domain to send from your server, etc...
If you have a non-Gmail email address, they receive topic notifications, as seen here:
I'm afraid, at the end of the day, this is not a software problem but a server problem with mail getting blocked or incorrectly configured. Sorry we don't have better news for you.