Outlook
Microsoft Outlook handles email with Cirrux over IMAP and SMTP, but most versions route your mail through Microsoft servers first.
Outlook reads your mail through Microsoft
Most versions of Outlook no longer connect to Cirrux from your own device. You give Outlook your app password, Microsoft stores it, and Microsoft’s servers sign in to Cirrux on your behalf and keep a copy of your mailbox. Microsoft documents this themselves.
Classic Outlook for Windows is the exception. It still talks to Cirrux directly. So do Thunderbird, Apple Mail and Cirrux on the web.
Which version you have matters
- Classic Outlook for Windows
- Connects to Cirrux directly and keeps your mail on your own machine. This is the version we’d point you to on Windows. Microsoft has said it plans to replace it, so this may not stay true.
- New Outlook for Windows
- Routes everything through Microsoft’s cloud, with no way to turn it off. It’s also where the sign-in problem below shows up.
- Outlook for Mac
- Routes through Microsoft’s cloud when you set the account up with Sign in. Some builds offer Sync directly with IMAP instead, which keeps the connection between your Mac and Cirrux. We can’t confirm that option is still there on the newest builds, so check it at setup.
- Outlook for iOS and Android
- Always routes through Microsoft’s cloud, with no opt-out. This has been the case since 2015, long before the Windows version changed.
“Invalid credentials” on new Outlook for Windows
New Outlook for Windows sometimes refuses to add a Cirrux mailbox and fails with this error, even when the password is correct:
INVALIDCREDENTIALS INTERACTIONREQUIRED
Three things to try, in this order:
- Make sure you’re using an app password. Your normal Cirrux password will not work in Outlook. Create an app password in your mailbox settings, under the Passwords tab, and use that instead. Check you created it on the right mailbox, which is the mistake we see most often.
- Sign in with a different alias. This is worth trying if the address was ever used with Microsoft before, for example as a work or school account in Microsoft Entra, or as a personal Microsoft account. We keep seeing this error on addresses with that history, and it stays attached to the address even after the Microsoft account is gone. Add an alias to your mailbox that has never been used with Microsoft, then set Outlook up with that alias and your app password. Mail sent to your original address still arrives in the same mailbox, and you can still send from it.
- Switch back to classic Outlook. Turn the New Outlook toggle in the top right of the window off. Classic Outlook connects to Cirrux directly, so it never makes the Microsoft sign-in that fails here. It won’t show your Cirrux calendar, though. Thunderbird is the other option on Windows, and it does sync your calendar.
If none of that helps: we can see the sign-in arrive at our servers and succeed, and then Outlook reports the error anyway. “Interaction required” is not an IMAP error. It’s a Microsoft sign-in error that happens inside Microsoft’s cloud, after we’ve already let them in.
This isn’t specific to Cirrux. People running other mail servers report exactly the same thing, and Microsoft has not published a cause or a fix. What we know from those reports:
- It sticks to one address. Removing and re-adding the account doesn’t help.
- A different address on the same mail server usually adds without any trouble.
- It isn’t two-factor, or the size of your mailbox.
- The same address keeps working in classic Outlook and Thunderbird.
If you’re stuck, mail us at help@cirrux.info and we’ll check our logs for your sign-in.
A note on calendars and contacts
Outlook handles email with Cirrux over IMAP and SMTP, but it does not support CalDAV or CardDAV. That means it can’t sync your Cirrux calendar, contacts or reminders. For those, use a CalDAV/CardDAV-capable app such as Apple Calendar, Apple Contacts, Thunderbird or DAVx⁵.