If you are utilising 2-Step Verification for added security to your account, when you're setting up this email account in VM Backup you'll need to generate an 'App password' specifically for VM Backup in order to be able to send email notifications.

Instructions on how to generate an 'App password' can be found here: 

Gmail: https://support.google.com/accounts/answer/185833

O365: https://docs.microsoft.com/en-us/azure/active-directory/user-help/multi-factor-authentication-end-user-app-passwords

For Gmail, ensure that you also, disable the "Less Secure Apps" option in this case too: https://support.google.com/accounts/answer/6010255

Important Note

Gmail and O365 use TLS, so ensure that the port number is 587