Encrypt Email works fine with iCloud accounts, you can setup an iCloud account on Encrypt Email as follow:
Personal Information Section
Description: write a description, "My iCloud Account" for example, or anything
Email Address: your iCloud email address (it will receive replays to your email)
Full Name: your full name or anything (t will be the sender that the recipient sees)
Sending mail Section
Outgoing Mail Server SMTP: smtp.me.com
Advanced sending options
Port: 587
Secure type: Transport Layer Security TLS
Authentication: Password
User Name: your iCloud email address
Password: your iCloud password
You can find more info about iCloud settings in the dedicated Apple's page:
https://support.apple.com/en-us/HT202304