Upwork is a work marketplace. Clients can post a job or browse pre-scoped services, review proposals, interview candidates, agree on hourly or fixed-price terms, and then manage the contract.
That flexibility is useful when you want to choose a specific person's portfolio, expertise, availability, and working style. It also means the result, timeline, price, revision process, and production scope depend on the freelancer you hire.
Mailiac is narrower. There is no talent search: the product scans your website, creates the newsletter, lets you request changes, and provides the HTML for your sending platform.