The message you are seeing is related to the API key that SmarterMail is trying to use to connect to Google, not the specific server that is making the connection attempt.
A few years ago, Google made using OAuth mandatory for interacting with their APIs and many of the email protocols. Using OAuth requires that the program uses a pre-registered API key that identifies the app when making the OAuth connection. To support this requirement, we included support for OAuth when configuring cloud service connections, email retrieval, and mailbox migration.
After that, Google added security analysis requirements for programs using certain APIs and protocols. The specific requirements depended on what APIs and protocols you were attempting to use, as well as whether your program is a desktop or web based application. Since SmarterMail is a web based application, the requirements for the security review are prohibitive. Fortunately, we were able to work around this limitation for a while by having users enable the "Allow less secure apps" setting in their Google account. Unfortunately, Google removed that setting from all accounts last year, leading to the message you encountered.
As far as we have been able to determine, there is no way for us to work around these restrictions. For this reason, we chose to remove Google integrations for Email Retrieval, Mailbox Migration, and Cloud Storage Connections from SmarterMail, beginning with the currently running beta.