Email tests you run with GlockApps don’t hurt your email deliverability because GlockApps doesn’t mark tested messages as ‘spam’, i.e. doesn’t show any negative user engagement.
The lack of user engagement affects only email placement with Gmail. We maintain two categories of Gmail email addresses: interactive and non-interactive. With interactive email addresses, GlockApps marks each received message as ‘seen’. With non-interactive Gmail addresses, GlockApps shows no user engagement. With that said, your deliverability to non-interactive Gmail addresses on the seed list can become worse.
If you observe an increase in the Spam placement rate with other mailbox providers over time, it means that the reputation of your sending domain and/or sending IP has dropped. The GlockApps seed list contains 70+ email addresses. This is a tiny quantity that could not impact your sender reputation. Mailbox providers take thousands of emails into account when calculating the domain/IP reputation.
The domain/IP reputation is calculated based on all the messages sent from the domain/IP. You may be sending email marketing campaigns to your email subscribers at the same time when you run email tests with GlockApps. If user complaints, spam trap hits, or bounce emails happen, they have a negative impact on your sender reputation which then impacts your deliverability.
And it is important to remember that spam filters are changing and adapting all the time. The same message may go to Inbox one day and to the Spam folder the other day.
You may also notice Inbox and Spam email placements with different email addresses with the same mailbox provider, for example, Gmail or Outlook. When a mailbox provider is unsure about your sender reputation (in case you have a new IP/domain or you haven’t sent from the IP/domain for a long time), it can take a random decision about the email placement.
Mailbox providers will never clearly show you what was the cause of a spam email placement. It is recommended to start the investigation by checking your domain/IP reputation. With Gmail, you can do it using the Google Postmaster Tools. With Microsoft domains, you can check the report in Smart Network Data Services (available only for dedicated IP addresses) to see the traffic coming from your IP address. Other mailbox providers don’t have a reputation check dashboard. GlockApps gives the links to the ISP postmaster or support pages under the Action Steps tab in the report.
Here are a few good articles that you can read to know how you can determine why your message went to the Spam folder: