Be aware that Mailbox.org allows any user to send emails as ("from") any other user via SMTP and these emails will look legit since they pass SPF and DKIM checks. Many consider this a security issue.
There was a quite lengthy discussion about this in their forum but they deleted it since. They refused to fix it. Archive.org still has it. Content is in German (sorry):
There was a quite lengthy discussion about this in their forum but they deleted it since. They refused to fix it. Archive.org still has it. Content is in German (sorry):
https://web.archive.org/web/20210123192856/https://userforum...