Last December, Google announced that they have be turning off less secure app (LSA) access to G Suite accounts, and that you should migrate to OAuth authentication instead. The first phase of the LSA turn-down was scheduled for June 15, 2020. As many organizations deal with the impact of COVID-19 and are now focused on supporting a remote workforce, Google minimize potential disruptions for customers unable to complete migrations in this timeframe. As a result, Google suspending the LSA turn-off until further notice. All previously announced timeframes no longer apply.

Last December Notice: https://gsuiteupdates.googleblog.com/2019/12/less-secure-apps-oauth...

Latest March Notice: https://gsuiteupdates.googleblog.com/2020/03/less-secure-app-turn-o...

Clearly Explained by Dr. Vimal Kumar in Koha geek Must Read all Kohaers http://kohageek.blogspot.com/2020/07/google-less-secure-apps.html

LSAs are non-Google apps that can access your Google account with only a username and password. They make your account more vulnerable to hijacking attempts. Instead of LSAs, you can use apps that support OAuth—a modern and secure access method".

How it affects Koha users?
After June 15, 2020 - Koha Users who try to connect to an LSA for Gmail with Exim or Postfix for the first time will no longer be able to do so. Users already connected can continue till February 15 2021.

After February 15, 2021 - Access to LSAs will be turned off for all G Suite accounts. Koha users can't send emails using LSA.

How to overcome this situation?
Google has introduced a new method to use Gmail service from third party software. The user needs to finish two-step verification for Google account. Then create an app password for Gmail clients (e.g. Exim, Postfix). Detailed instructions in the following link,
https://support.google.com/accounts/answer/185833?hl=en