If you use GSuite from Google for outgoing mail in WHMCS Google will no longer support mail connections from WHMCS with a username and password only starting on February 15, 2021.
Access through only a username and password makes your account more vulnerable to hijacking attempts. Moving forward, only apps that support a more modern and secure access method called OAuth will be able to access your G Suite account.
To continue using a specific app with your G Suite accounts, users in your organization must switch to a more secure type of access called OAuth. This connection method allows apps to access accounts with a digital key instead of requiring a user to reveal their username and password. We recommend that you share the user instructions (included below) with individuals in your organization to help them make the necessary changes. Alternatively, if your organization is using custom tools, you can ask the developer of the tool to update it to use OAuth. Developer instructions are also included below.
Access to LSAs will be turned off in two stages:
June 15, 2020 - Users who try to connect to an LSA for the first time will no longer be able to do so. This includes third-party apps that allow password-only access to Google calendars, contacts, and email via protocols such as CalDAV, CardDAV and IMAP. Users who have connected to LSAs prior to this date will be able to continue using them until usage of all LSAs is turned off.
February 15, 2021 - Access to LSAs will be turned off for all G Suite accounts.