The two get shopped together constantly, and they're different tools. Mailtrap's sandbox catches your app's outbound SMTP so you can inspect it before it reaches anyone. MailFixture hands your test suite real receiving inboxes and asserts on what actually landed. Here's how to tell which one you're looking for.
Mailtrap's sandbox answers: "does the email my app is about to send look right?" You point your app's SMTP credentials at their trap; outbound mail lands in a shared sandbox with HTML checks and spam analysis instead of reaching real people. It's send-side QA, and it's good at it. On paid plans each sandbox also gets a real inbound address on their fixed inbox.mailtrap.io domain, and they now sell a separate Inbound Email product — but the sandbox's core is still the trap.
MailFixture answers: "did the email actually arrive, and what was in it?" Your suite creates an inbox at a real address on a real MX-routed domain, drives your app, and long-polls for the result — through whatever provider your environment really sends with. Nothing gets rewired, which is why the same test runs against localhost, staging, or a preview deploy, and why it catches the config drift a trap can't see.
Plenty of teams legitimately run both: trap outbound in staging so customers never get test mail, receive-side assertions in CI for the flows that matter. If one budget line has to win, pick the one that matches the question your tests ask.