Currently, if an external user to Azure AD trying to long with crowd,
the user will not able to login and below's error message will show in the logs:

2018-02-01 12:42:12,240 http-nio-127.0.0.1-8095-exec-4 ERROR [console.action.application.ViewApplication] could not authenticate user test001_outlook.com#EXT#@ <tenant-name>.onmicrosoft.com0
com.atlassian.crowd.exception.InvalidAuthenticationException: could not authenticate user test001_outlook.com#EXT#@<tenant-name>.onmicrosoft.com0

If will be nice that if crowd supported with the AAD external user as wel

The #EXT# in the reported username indicates that this is a user that's external to the AAD, and currently does not supported with Crowd