this is not a mailinabox issue, it’s a gmail issue. As with any mail servers you set up, regardless whether it’s mailinabox or you install postfix from stretch, you have to spend some effort building up reps, or getting your IP unblock with some providers (e.g. hotmail, livemail, yahoo). You wouldn’t have problems with typical business mail servers hosting personal/business domains.
It’s not difficult to build reputation. As long as you send legitimate emails, you will eventually build up enough reputation on your domain and IP and your emails will stop going to the spam folder.
Remember… don’t change your IP once you start your box else you gonna do it all over again.