The Emails are Ending up in Spam
There are many reasons a mail will end up in a spam folder and none are really down to Post Gopher, more how your website/server is configured.
For example, if your domain on which Post Gopher is running is called pgdomain.com, then if the server is configured to send mail from this domain, only mail from firstname.lastname@example.org will be sent and possibly delivered to the inbox (see later). IF you try and enter the email address anyname@anotherPGdomain.com and send an email, then Google will see the email coming from anotherPGdomain.com BUT sent from a domain named pgdomain.com. This will flag things up and end up in spam.
Now when it comes to verifying email and inbox delivery, then there are things like SPF, DKIM etc etc which are all to do with validation and again nothing to do with Post Gopher and quite complex to setup and you need to speak to your hosting company to find out if email is configured correctly on your domain.
We use https://wordpress.org/plugins/wp-mail-smtp/ which allows you to use gmail, for example, to sent out emails from WordPress.
Note: the WP-SMTP plugin is not so much "outdated" as not needing updating! Plugins are updated either due to security breaches or bugs being found, or not working with updated versions of WordPress itself. Since none of these apply to this plugin, you can safely use it.