If you want to use your existing GSuite / Google hosted Email account to send calendar invites, please read this to understand the pre-requisites required to integrate the same.
Perquisite -
1) Turn ON Less secure app access for User's account you wish to use to send calendar invite's from (https://myaccount.google.com/lesssecureapps?pli=1&rapt=AEjHL4NBafVsGV2Nfl_vfo5MQsToVW2lMyh17ZegI4UxaJq3hiXsUcFVydUO9n9jp6uJ8M-sOpzmNu3JHirVblzsjPsdWPS_eQ)
2) Turn ON POP Access on User's account (https://mail.google.com/mail/u/0/#settings/fwdandpop)
3) Turn OFF - 2FA Authentication (https://myaccount.google.com/signinoptions/two-step-verification/enroll-welcome)
Step 1 - Less Secure Apps - ALLOW Access
1) Go to the Less secure app access section of your Google Account. You might need to sign in.
2) Turn ON "Less secure app access" for your user's account
Note: If you see a message that you as a user do not have access to turn ON or OFF this setting, you may need to contact your IT Manager to help you manage this or Login to your Google Admin Control Center and trigger this setting from there. Follow below steps if you need to do this and have access to your Google / GSuite Admin Tools -
Manage User's access via Google Admin Panel
Step 1.1 - Goto https://admin.google.com/ac/home and login to your admin account
Step 1.2 - Goto ISA security settings - https://admin.google.com/ac/security/lsa
Step 1.3 - From here you can either trigger "Allow users to manage their access to less secure apps" OR click on "Users" in left menu and enter the email address of user who's access details you wish to change.
Step 1.4 - Grant access to the user
Step 1.5 - Save Settings
Step 1.6 - Logout as the "Admin" and go to the Less secure app access section of your Google Account you wish to use for sending calendar invites. You might need to sign in.
Step 1.7 - Turn Allow less secure apps - ON.
Step 1.8 - Save and Exit
Ensure you see below screen when you visit your Account's Security Settings Window -
Step 2 - Enable POP settings
Step 2.1 - From mail.google.com (after login) click on Settings (Gear icon) and goto See All Settings
Step 2.2 - Click on "Forwarding and POP/IMAP"
Step 2.3 - Click on "Enable POP for all mails" and "Enable IMAP" on your account
Step 2.4 - Save and Exit
Step 3 - Turn 2FA Authentication OFF
If you use 2FA Authentication on your account, then SMTP application wont' work. It's recommended that you turn OFF 2FA authentication on your account.
Step 1 - Goto your Google Account Security Settings Section (https://myaccount.google.com/security?rapt=AEjHL4NBafVsGV2Nfl_vfo5MQsToVW2lMyh17ZegI4UxaJq3hiXsUcFVydUO9n9jp6uJ8M-sOpzmNu3JHirVblzsjPsdWPS_eQ)
Step 2 - Scroll down to "Signing in Google" section - and select 2 Factor Authentication - https://myaccount.google.com/signinoptions/two-step-verification/enroll-welcome?rapt=AEjHL4NBafVsGV2Nfl_vfo5MQsToVW2lMyh17ZegI4UxaJq3hiXsUcFVydUO9n9jp6uJ8M-sOpzmNu3JHirVblzsjPsdWPS_eQ
Step 3 - If this is turned ON - Please turn it OFF. If this is OFF - Keep it as it is.
Verify your account settings are displayed as below -
Now your account is configured to allow SMTP connections to send out emails as YOU.
Step 4 - Final Account Verification
Verify your Security Settings to confirm your account is setup to allow SMTP applications like Let's Calendar to access and send calendar invites. Please bookmark and save all windows you want to visit / open again, including Let's Calendar and this help article. Now Log out of all your Google Accounts, Gmail and all Google Services, Close your browser and open it once again to ensure all settings were previously saved and are reflecting in your Google Account.
1) Click on your account/profile icon from Gmail, on top right corner and click on "Manage your Google Account" - https://myaccount.google.com/?tab=kk&rapt=AEjHL4NBafVsGV2Nfl_vfo5MQsToVW2lMyh17ZegI4UxaJq3hiXsUcFVydUO9n9jp6uJ8M-sOpzmNu3JHirVblzsjPsdWPS_eQ
2) Click on "Security" from the left side menu - https://myaccount.google.com/security?rapt=AEjHL4NBafVsGV2Nfl_vfo5MQsToVW2lMyh17ZegI4UxaJq3hiXsUcFVydUO9n9jp6uJ8M-sOpzmNu3JHirVblzsjPsdWPS_eQ
3) Scroll down to Signing in to Google section and ensure the following -
3.1 Use your phone to sign-in is set to OFF
3.2 2-Step Verification is set to OFF
4) Scroll down to "Less Secure App Access" and confirm that this is set to ON (It should show a warning sign next to ON)
Note: Do not close this window. For now you have made all necessary settings, to try to complete SMTP configuration of Let's Calendar, however Google sometimes detects unrecognized login attempts and still blocks the same. You may need to revisit this section to allow Let's Calendar access.
You can proceed with the next step in this guide.