Office 365 users in your organization who connect to Exchange Online by using incorrect hardcoded service settings (or IP addresses) will be unable to connect to the service after these outdated service settings are discontinued on July 8, 2015. (The original date was June 26, 2015).
The services for which users may be using incorrect settings include the following:
Autodiscover: A feature that's used to automatically discover server names for connectivity
Exchange ActiveSync (EAS): A protocol that's used by email client applications on mobile devices
POP3 and IMAP4: Protocols that are used by email client applications on desktop and mobile devices
SMTP: Protocol that's used for sending email messages
If users continue to use the outdated, hardcoded service settings, they will be unable to connect to the service after July 8, 2015. Use the information that's provided in this article to make sure that these settings are set correctly for users in your organization.
The following table lists the features, protocols, and expected server settings that must be set, and provides links to the articles that contain instructions to configure these settings.
Windows uses a file that's named Hosts to map hostnames to IP addresses. This file is checked before DNS is used to resolve a server name. If the Hosts file on a Windows computer has an entry for any of the server names in the table, and if the entry is associated with a hardcoded IP address, DNS is not used to resolve that server name. Make sure that you reset the Hosts file on all such systems that may be experiencing the issue that's documented in this article.
For more information about how to reset the hosts file, see the following Microsoft Knowledge Base article:
972034 How can I reset the Hosts file back to the default?