Sonic.net

Although mail.app has been working as a Sonic client for years on a Mac running El Capitan, and although it was easy to get the built-in mail client working on a new iPhone 7, mail.app fails to start working on a new Mac running Sierra. It always reports that either the user name or the password (it won't say which) is incorrect. Diagnosing the problem is made more difficult by the fact that the current Mac OS is 10.12, while the latest Sonic configuration instructions for the Mac mail client are based on OS 10.9. The 10.12 interface is not the same as that of 10.9, so the user has to guess how to translate Sonic's instructions into the current interface.

After a couple of hours of Apple support, this has been escalated to engineering, but I have also found a solution, not recommended by Apple. It was to create a new account in mail.app. That was quick, painless, and successful.

Thanks for sharing the fix, Utilika!

Strangely, it also turned out that a successful addition of a mail account can become a failure again when the Mac notifies you that an account has been added. If you approve that addition, the account gets corrupted. It remains viable only if you answer "Not now".