Gmail considers regular email warm-up programs and backups (i.e., Hotter, Outlook, etc.) to be "less secure", so in order for them to get access to your account, your "allow less secure apps" must be enabled.
Use this link to easily enable Less secure apps access.
It takes months for new applications like Folderly to go through all the Google verifications procedures, even though we are in the middle of doing this, we need our clients to have access to Folderly to benefit from our spam fixing features. That is why we are using "less secure apps" access for the time being.
How to enable/disable "less secure apps" access?
Go to you your Google Admin console -> https://myaccount.google.com/
Go to the Security section -> https://myaccount.google.com/u/1/security
Disable your 2-Step Verification. Read this guide to enable/disable 2-Step Verification http://help.folderly.com/en/articles/4507196-2-step-verification-how-to-enable-disable. (This setting is not available for accounts with 2-Step Verification enabled. Such accounts require an application-specific password for less secure apps access.)
Once you disabled your 2-Step Verification go to Less Secure app access section -> https://myaccount.google.com/lesssecureapps
5. If less secure app access is disabled, please turn it on.
If Less secure app access option is unavailable
The Less secure app access option may be unavailable if Google Workspace Admin disabled users ability to allow less secure apps.
To re-enable access:
Once the "less secure apps" access is activated, Folderly platform would have access to only receive and send "warm-up emails" from and to the designated account.