iOS11 + EAS + native email client: iOS11.1 will solve it
iOS11.0 didn´t work with Outlook.com, Exchange 2016 and O365 (our article). 11.0.1 seemed to fix this issue (Apple Support), but not for all environments. The main problem seems to be the handling of SSL/TLS version handling of iOS11 + SHA-1 certificates + CBA (Cert Based Authentication). Since iOS10.3, SHA-1 certificates …