Was using protonmail, and when Flarum sent an email I would be able to see the sent email in my outbox.
I'm testing out a different provider, and although only the server name and login credentials have changed in the Flarum config (port is still '587' and encryption is still 'TLS'), I do not see the sent email in the outbox of the new email provider's web-app. The email itself is sent just fine though.
Additionally, when I send an email using SMTP from Thunderbird on my laptop, it successfully shows up in the outbox on the email provider's web-app. So it's definitely possible to make it work!
I'll try and reach out to the new email provider as well, but I thought I'd ask here to see if anyone has a suggestion. This is with Flarum v.1.8.10. Thanks!