ActiveSync needs to be enabled on your mailbox for these instructions to work properly. Please open a support ticket if you are unsure if ActiveSync is available and enabled for your mailbox.
On your iPhone, open the Settings.
Tap Add Account.
Tap Microsoft Exchange.
Initially, fill out the following fields:
Email -- your full email address.
Description - A friendly name for the account, such as your company name, "Work Email" or whatever.
You may be prompted with a screen that asks "Sign into your "domain.com" Exchange account using Microsoft?" -- If so, choose the "Configure Manually" option. Otherwise, you will be asked to sign in with a Microsoft School or Work account, or a personal Microsoft account.
You'll be prompted for your account password.
After providing your password, you'll be asked to provide the Server (mail.activesend.com), Domain (leave blank), and Username (your email address).
Clicking Next will bring up the options you want to sync: Mail, Contacts, Calendars, Reminders and/or Notes. Select the items you want to sync.
Hit the Save button.