IPhone
From XMission Wiki
Note: If you're going to be using your email on computers or devices as well as your iPhone or iPod touch. XMission strongly recommends that you change the settings on your computer to check email using IMAP instead of POP3. This allows you to access your email from multiple devices and webmail wherever you are in the world. Please refer to our setup guides for instructions on how to configure your email client for IMAP access or contact our Technical Support line 24 hours a day for a walkthrough.
iPhone setup If your email address ends with @xmission.com or you use our virtual mail service click here.
Zimbra iPhone setup If you are using our Zimbra hosted mail service click here.