A test sent from somewhere else says little about a Mailchimp campaign. Half of what decides the folder is added on the way out: the servers Mailchimp sends from, the domain that signs the message, the tracking it rewrites into your links. So the test here is sent by Mailchimp, from your account, with your settings.
Testing a draft
Connect your account above, pick a draft and press Test. Mailchimp sends a test email of the campaign to one of our addresses, and the report fills in within a minute: the score, SPF, DKIM and DMARC with alignment, 21 blocklists, the links, the HTML, the Gmail and Yahoo bulk-sender rules, and a fix plan with DNS records ready to paste.
Tick the provider box and the same test also goes to inboxes at different mailbox providers, so the report shows whether it landed in the inbox, in promotions or in spam at each of them. Mailchimp only sends test emails with the campaign's own subject, and those inboxes find a test by a marker in the subject. That is why the test goes out from a copy of your draft, with the marker added to the copy's subject. The copy is deleted straight after, and your draft is never edited.
Mailchimp limits test emails per campaign (twelve on the free plan, seventy on a paid one). On the free plan the provider check therefore covers eleven inboxes, spread over as many providers as that allows, and the report shows only the inboxes the email actually went to.
Watching an audience
A single test tells you about one email on one day. For everything after that, add a permanent address to an audience. It joins as an ordinary subscribed contact with the tag "Email Spam Tester", so you can find it, and every campaign and automation email the audience gets is checked when it arrives. The letters line up on this page as a series, with the problems that keep coming back from one email to the next.
The address is a real contact, so it counts toward your plan's contact limit like any other. Remove it with the Stop button and the contact is archived in your audience, which keeps its history in Mailchimp. Disconnecting the account removes every permanent address it added.
What the connection allows
Email Spam Tester reads your drafts, audiences and campaign reports, makes and deletes the test copies, and adds or archives its own contact. It does not send campaigns, does not touch your other contacts and does not change your drafts. The access token is stored encrypted and dropped when you disconnect. Mailchimp keeps its side of the connection until you also remove Email Spam Tester from the apps connected to your account in Mailchimp itself. What is kept, and for how long, is in the privacy policy.