NETx Server settings for messenger plugin:

From: <your Yahoo email address>
Host: smtp.mail.yahoo.com
Port: 587
UseAuthentication: T
Username: <your Yahoo email address>
Password: <app password>
- This is NOT your Yahoo password. See details below
UseSsl: T

In order to use a Yahoo account to send notifications from NETx Servers, you need an app password. App passwords are only available if 2-Step authentication is activated for your account. Thus, ensure that you have activated 2-Step authentication. If not yet, please activate it in your account settings.

Afterwards, you have to generate an App Password for your server:

In Browser log into your account
Navigate to https://login.yahoo.com/myaccount/security/
Scroll to "Other ways to sign in"
Press "Generate and manage app passwords"
Give your password a name
Click "Generate password"
It will give you 16 chars token.
User this the 16 chars token as a password in the Password field in Email definitions

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