A welcome email has one job before it has any others: tell the subscriber what they have joined, why staying subscribed is worth it, and what to do next. Everything else is optional.
That sounds obvious, and it is still the part shops skip. The default output from most platforms is a receipt. It confirms an email address was captured, which is a fact about your database rather than a message to a person.
What earns the slot instead is specific: your best three products and why they are the best three, how the service actually works, the answer to the question you get asked most, or simply proof that a human runs the place. Whatever you promised at the signup form should be delivered here, in the first email, while the promise is still fresh.
What I do about it
The signup promise and the welcome email get written at the same time, by the same person, so they cannot contradict each other. If the form says early access, email one contains early access.