Hi hyung,
If you’re having trouble with emails getting marked as spam, I would suggest using a plugin like WP Mail SMTP, which lets you route emails via your real email address/server, which should decrease the likelihood of them being flagged as spam, etc.
Please note, for anyone else who comes across this, that the default notifications sent to the customer when making a booking are part of the free plugin and not at all related to the fact that this user is also using the premium version.
That being said, if you are having an issue with a notification feature that is specific to the premium version, then you will need to contact us via email or our support center (https://www.fivestarplugins.com/support-center/), as discussion of the premium version is prohibited here (https://www.ads-software.com/support/guidelines/#do-not-post-about-commercial-products).