Today my father has installed Aquamail, he has only one Gmail account, but failed to do so.
Then I tried it on my smartphone, error loging, a very long message saying to check the login credentials (username and password).
I tried to access his account from my PC and everything worked ...
Then I read an email came to him, in which Google stated that "We recently blocked an attempted access to your Google Account" ...
Then I opened the Gmail settings to remove the protections that were active.
After this intervention, it was possible to set up Gmail on Aquamail.
With this example I would suggest if there is a way to inform the user of what is the cause that does not allow you to add a Gmail account, because inexperienced users could do as my father, uninstall Aquamail, and do not try anymore.