You might not be able to send email with an Outlook.com, Office 365, or Exchange account until you update to iOS 11.0.1.
If your email account is hosted by Microsoft on Outlook.com or Office 365, or an Exchange Server 2016 running on Windows Server 2016, you might see this error message when you try to send an email with iOS 11: “Cannot Send Mail. The message was rejected by the server.”
To fix the issue, update to iOS 11.0.1 or later.
Action Required by: October 31, 2017 at 5:59 PM UTC
Details: On October 31st, 2017, Exchange Online mailboxes in Office 365 will require connections from Outlook for Windows use MAPI over HTTP, our new method of connectivity and transport between Outlook for Windows and Exchange. In May of 2014, Microsoft introduced MAPI over HTTP as a replacement for RPC over HTTP. RPC over HTTP was a legacy connection protocol that is being deprecated from Exchange Online. Beginning October 31, 2017, Outlook for Windows clients using RPC over HTTP will be unable to access their Exchange Online mailbox. The necessary action depends on the version of Outlook in use in your organization. If you are using Outlook 2007 or earlier, you need to upgrade. Outlook 2007 does not contain support for the MAPI/HTTP protocol. We encourage you to update to the Office 365 ProPlus subscription, or access Outlook via the web browser (which is included in your current subscription plan). Outlook 2010-2016 customers will need to ensure their version of Outlook for Windows is set up to support MAPI/HTTP. At a minimum, you should ensure you have installed the December 2015 update. Lastly, ensure your Outlook clients are not using a registry key to block MAPI/HTTP.
Message Center: MC85988 – Potential service disruption for Outlook for Windows users
Posted: November 16, 2016
Additional Information: KB3201590: RPC over HTTP deprecated in Office 365 on October 31, 2017