To setup your Hosted Exchange 2019 mailbox on your iPhone for iOS18, follow these instructions:

Navigate to your iPhone “Settings”
Scroll down and then select “APPS”
Scroll down and then select “Mail”
Select “Accounts”
Select “Add Account”
Select “Microsoft Exchange”
Enter the E-mail Address and a Description for the account
Select Configure Manually
Enter the Password associated with the mailbox
In the “Server” field enter the following: exchange.infoquesthost.com
Leave the “Domain” field blank
In the “Username” field enter the E-mail Address
Select “Next”
Select which items you’d like to sync with your device – Mail, Contacts, Calendars, Reminders
Select “Save”