Emails and attachments sent by Outlook for Android and iOS all pass through Microsoft's servers. The behaviour is problematic for companies that run a local Exchange server and which restrict their employees from using third party servers.
The Outlook for Android and iOS application stores all email, including attachments, on Microsoft servers and then sends it to the receiver. The reason that the Outlook apps send mail through Microsoft's servers is because the apps are based on Acompli, an e-mail app which Microsoft recently acquired.
Although Microsoft has changed the name and the logo of the app, it is exactly the same as the Acompli app. Both Acompli and Outlook for Android and iOS mention in their terms and conditions that emails pass Microsoft's servers.
Microsoft states the app meets "internal borders" in the area of privacy. The company also underlines that Exchange administrators can block access to the app.
