Update [April 20, 2017]: As you already know, the Nine mail app supports multiple accounts. But, unfortunately, until the recent update, there was no way to change the signature for each account individually. However, thanks to the update that has pumped the version to 3.2.5, it now introduces multiple signatures. You will find a “signature” setting in each of your nine mail accounts.
To change the signature for each account separately go into Nine mail settings, then select your account and tap the “signature” option.
Moreover, the latest update allows you to add a GIF to your emails from the Google keyboard, GBoard. And the update also fixes the ordering issue that was present in the calendar widget.
The Nine mail app is a business email app developed by 9Folders Inc. The app is not cloud based rather it connects to the actual mail servers and stores the passwords and emails on your device. Moreover, full of features, the UI of the app is also intuitive.