When using an iCloud email address, you can connect this with Octoa to send out emails from the Octoa tool. Therefore, you must use the SMTP settings under the email settings and ensure you generate an app-specific password within your iCloud settings.
How to generate an app-specific password
Sign in to you Apple ID
Under Sign-In and Security you choose App-Specific Password
You create an App-Specific Password for Octoa
Copy the App-specific password that is provided
Paste the App-specific password under the SMTP settings of Octoa https://app.octoa.com/settings/email
Host: smtp.mail.me.com
Port: 587
Username: name@icloud.com
Password: App-specific password
When you click 'Send a test email' and a successful connection is made, it will switch from off to on.
Don't forget to click on save at the bottom of the page.
More information can be found here: https://support.apple.com/en-us/HT204397