Recalling an email in Outlook, does it actually work?

Even inside the same company it only works on mail they haven't opened yet. Once they've read it, recall can't touch it. On Microsoft 365 you at least get a report email afterwards saying which recipients it worked for, so you're not left guessing.

Only in a narrow case. Both of you have to be on the same Microsoft 365 / Exchange organisation, basically colleagues at the same company. Send it to a Gmail or Yahoo address and recall does nothing, they keep the original and some get a second email saying you tried to recall it, which is worse. The actual fix is to stop it before it leaves. Outlook has an undo send option in the settings that holds mail for a few seconds, and in classic Outlook you can set up a rule to delay all outgoing mail by a minute or two. That has saved me more times than recall ever has.