The first hour
1. Sign up to your own list.
Use an address you don't normally use, a personal Gmail is fine. Then leave it alone and come back.
2. Did anything arrive?
If nothing came, you've found it. Stop here and go fix that one thing.
3. How long did it take?
Under a minute is right. A few hours means something's batching that shouldn't be. A day means it isn't automated at all.
4. Who is it from?
A person's name, or no-reply@? People reply to names. Nobody replies to no-reply, which is the point of it, and that's usually a decision nobody made on purpose.
5. Where did it land?
Primary, Promotions, or Spam. Promotions isn't a failure, it's a signal about your content. Spam is a failure.
6. Does it say what happens next?
Or does it hand over the thing they asked for and stop? A first email that ends without a next step is a conversation you started and then walked away from.
The first week
7. Is there a second email?
When does it turn up? Did someone decide that, or did they set it once and never look again?
8. Does the sequence end, or does it just run out?
There's a difference, and only one of them was designed.
9. What if they buy halfway through?
Does the sequence notice, or do they keep getting sold the thing they already own? That's the fastest way to look like nobody's home.
The mechanics
10. Six months from now, can you tell where this person came from?
If every subscriber lands in the same bucket, every decision you make about them later is a guess.
11. Can someone be in two sequences at once?
Try it. Sign up for two things with the same address and see what turns up.
12. Someone's on three of your lists and wants one email a week. What's the rule?
Most lists don't have one. They find out when somebody unsubscribes and tells them why.
The plumbing
13. SPF, DKIM and DMARC. Do you have all three?
Two minutes to check. Open an email you've sent, view the original, and find the Authentication-Results line. You want three passes.
14. One-click unsubscribe?
Not the link in the footer, the one in the headers. Gmail and Yahoo expect it from anyone sending in volume, and it's easy to miss because your footer link makes it look handled.
15. When someone replies, where does that go?
And does a person read it? Replies are the best signal a list produces and usually the least watched.
The two honest ones
16. When did you last send?
And did you decide that, or did it just happen?
17. If you vanished for two weeks, would anything go out?
Not could somebody technically do it. Would it happen.
Found three or more?
That's not a content problem. It's an ownership problem. Email works when somebody's paying attention to it and quietly stops when they're not, which is normal, and fixable, and almost never about writing better subject lines.
Or find your biggest email leak, with a live check of your sender records.