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: