If you have or are setting up a Microsoft Exchange server, see the iOS Deployment Technical Reference Guide for further information regarding the required configuration of Exchange for iOS access to Mail, Calendars, and Contacts.
The following information, from Microsoft's TechNet Library, may also be useful when setting up or configuring Exchange servers for iOS mail access:
For Exchange 2013 documentation:
For Exchange 2010 documentation:
For Exchange 2007 documentation:
For non-Exchange mail server configurations, or for Exchange server access via IMAP, use the following settings to enable secure iOS access:
Open port 993 to allow email to be received through the firewall.
The proxy server must be set to IMAP over SSL only. SSL ensures that mail is securely encrypted during wireless transmission.
As a best practice and for additional security protection, install a digital certificate on the server from a trusted certificate authority such as Verisign.
Installing a certificate from a certificate authority (CA) is an important step in ensuring that your proxy server is a trusted entity within your corporate infrastructure.
Either port 587, 465, or 25 must be opened to allow email to be sent from iOS.
When sending a message, iOS automatically checks first for port 587, then 465, and then 25. Apple recommends opening 587 as the most reliable, secure port because it requires user authentication. Port 25 is considered to be the least secure because it's been around the longest and is subject to more attacks by hackers. It's also the port that some ISPs block by default to prevent unsolicited spam.