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

Instructions on how to generate an 'App password' can be found here: https://support.google.com/accounts/answer/185833

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

Important Note

Gmail uses TLS, so ensure that the port number is 587