Previewing a message in the browser tells you the variables resolved. It does not tell you how it looks in Outlook, whether the unsubscribe footer renders, or whether the sending mailbox is configured correctly at all.
You can now send a test copy directly from the campaign, choosing which mailbox it goes from, to any address you have verified. The test goes through the real sending path — same mailbox, same rendering, same footer — so what you receive is what a prospect would receive.
Failures report the mail server's actual error, which makes this the fastest way to confirm a mailbox works before a campaign depends on it.