Email deliverability testing only tells you the truth if you’re checking the right addresses. If you’re serious about making sure your emails really reach the inbox—not just in theory, but for your actual audience—you need a custom seed list. The default lists in most tools are too generic to be useful for everyone. This guide walks you through building a custom seed list in Glockapps, with a focus on practical steps, not vendor marketing fluff.
Whether you run a small newsletter, a SaaS product, or just want to keep your boss off your back about open rates, this is for you.
Why bother with a custom seed list?
Before we get to the how-to, let’s be clear: using the built-in seed lists from deliverability tools like Glockapps is easy, but it’s rarely accurate for your unique audience. Here’s why:
- Default lists are generic. They’re made up of random test addresses, not the mailbox providers (or geographies, or setups) your real subscribers use.
- You probably don’t care about every provider. If your list is 90% Gmail and Apple Mail, why stress about Yandex or T-Online?
- Some mailbox setups matter more. Corporate addresses, Google Workspace, Outlook with custom domains—all behave differently than consumer Gmail or Yahoo.
A custom seed list gets you closer to reality. You test your mix of ISPs, regions, and mailbox types, not some random blend.
Step 1: Figure out which email providers actually matter
Don’t guess—look at your actual data. Here’s how to get started:
- Download a recent export of your subscribers from your email platform.
- Sort by domain. In Excel or Google Sheets, extract what comes after the
@in each address. - Count up the top providers. Usually, Gmail, Yahoo, Outlook/Hotmail, and Apple (icloud.com, me.com) come up. But check for corporate domains, too—especially if you’re B2B.
Pro tip: If you send B2B emails, include a few addresses on Google Workspace, Office 365, and maybe your own company’s domain. These can behave differently than consumer mailboxes.
Focus on the top 5-10 domains that make up 80%+ of your list. Ignore fringe providers unless they’re mission-critical.
Step 2: Create test inboxes for your key providers
This is the part that takes some manual work, but it’s worth it.
- Create new inboxes at each target provider. For Gmail, Outlook.com, Yahoo, etc.—just sign up for free consumer accounts.
- For business-focused platforms (Google Workspace, Microsoft 365), spin up a free trial or use spare seats if you already pay for these services.
- For custom/corporate domains, ask a friend or IT person if you can use a real inbox, or set up a test mailbox on your own domain.
Keep a spreadsheet with your test addresses, login info, and what provider or setup each one represents. You’ll need this for Glockapps and for checking your results.
What not to bother with: - Don’t waste time creating accounts at obscure providers unless you see a lot of subscribers there. - Don’t add addresses you can’t actually check (e.g., someone else’s inbox).
Step 3: Build your custom seed list file
Glockapps lets you upload your own list of test addresses. Here’s how to prep it:
- Make a plain text file (.txt or .csv).
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List one test address per line. Example:
yourtest1@gmail.com yourtest2@yahoo.com yourtest3@yourdomain.com
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Optional: Add tags or notes in your spreadsheet (not in the file) so you know which address is which.
Keep this list up to date. If you lose access to an inbox, replace it. Clean out test addresses that go stale.
Step 4: Upload your seed list to Glockapps
Here’s the practical part—actually getting your list into Glockapps.
- Log in to your Glockapps dashboard.
- Go to “Tools” or “Seed Lists.” The UI changes now and then, but you’re looking for anything labeled “seed list,” “custom list,” or similar.
- Choose “Create new seed list” or “Add custom list.”
- Upload your file or paste your addresses into the form.
- Name your list something clear, like “Main audience test – June 2024.”
Glockapps will give you a set of instructions or a “test ID” for sending to these addresses. Follow them exactly.
Watch out for: - Some providers (especially Microsoft and Google) are really strict about new accounts. You may need to warm up addresses or solve captchas. - Glockapps may limit the number of custom lists, depending on your plan.
Step 5: Run a deliverability test using your custom seed list
The point is to see how your real-world emails perform—not just the default Glockapps message.
- Send your actual campaign (or a close copy) to your custom seed list. Don’t just use Glockapps’ default test message.
- Wait—don’t open the inboxes right away. Give ISPs an hour or two to sort your message.
- Log into each test inbox and check:
- Did your email land in the inbox, promotions tab, spam, or nowhere?
- Is your display name and subject line showing correctly?
- For Gmail: Did the message skip the Primary tab?
- Record your results in your spreadsheet or in Glockapps’ interface.
Pro tip: Run the test more than once, especially if you change send times, subject lines, or content. Deliverability can vary a lot.
Step 6: Interpret your results—don’t panic over every red flag
Here’s the honest truth: no seed list test is perfect. A few things to keep in mind:
- Seed tests are a proxy, not gospel. Real subscribers have years of engagement history—your test inboxes don’t.
- One-off spam folder hits happen. Occasional failures aren’t the end of the world.
- Watch for patterns. If Gmail always dumps you in spam, you’ve got a real problem. If it’s random, it might not matter.
- Don’t obsess over 100% inboxing. Even the best senders see the odd message miss the main inbox.
Use your seed list to spot big problems, not to chase perfection.
Avoid these common mistakes
- Using only the default Glockapps seed list. That tells you how you perform in general, not with your actual audience.
- Testing with a totally different message. Always use your real campaign, with the real subject and content.
- Letting your seed list go stale. Update test inboxes if you lose access or if your audience mix shifts.
- Ignoring engagement. Seed inboxes don’t behave like real users—they won’t open, click, or reply.
Keep it simple—and iterate
You don’t need a massive, complicated seed list to get value. Start with the main providers your real subscribers use. Test regularly, but don’t let testing paralyze you. If you see consistent problems, dig deeper; if not, keep sending good email and focus on your content.
Building and maintaining a custom seed list in Glockapps takes a little effort, but it pays off in more reliable, actionable deliverability data. Don’t overthink it—just start, and tweak as you learn.