Step 1. Generate a third-party app password in your Yahoo account:

• Sign in to your Yahoo account and go to your Account security page.
• Click Generate app password or Manage app passwords.
• In the drop-down menu, select Other App, provide a name  e.g. Acronis) and click Generate. Copy or write down the generated password

Step 2. In Acronis True Image notifications settings specify:

Check SMTP authentication box.

SMTP server: smtp.mail.yahoo.com
Port: 587
Encryption: TLS

In SMTP authentication settings specify your Yahoo e-mail and the third-party app password generated at Step 1 (not your e-mail password). 

E-mail notifications should now be sent successfully.

Step 1: generate a unique app password as described here:

• Go to the Security basics page and sign in to your Microsoft account.
• Select More security options. 
• Under App passwords, select Create a new app password. A new app password is generated and appears on your screen.

Step 2:

• For any of these e-mail services specify the following Acronis True Image notifications settings:
  SMTP server: smtp-mail.outlook.com
  Port: 587
  Encryption: TLS
• In SMTP authentication settings specify your e-mail and the app password generated at step 1.
  

E-mail notifications should be sent successfully.