If you are using Google suite or Office 365 and have 2-factor authentication enabled, you need to connect your email account via SMTP/IMAP servers. 

When connecting the email account, enter the app-generated password in the SMTP details. 

Here's how you can generate an app password for Klenty:

A. Generating app password if you are using Google suite

B. Generating app password if you are using Office 365