If you compare the authentication settings between your Firefox OS device and another device that is able to send mail through Zoho, can you see any problems there, for example, are there fields or settings missing from the Firefox OS mail client that are present and populated on your other device?

If I look at my thunderbird, yes, there is an option missing: Authentication method. All the other options are in my Firefox OS, so it could be this. In Thunderbird I have the option "autodetect" and it choosed "normal password".

Unfortunately, I don't have an Firefox OS device to experiment with. When I look at the screen shot for adding an account (Add email accounts), I see a "Manual setup" option, but I don't know whether that exposes the SMTP Auth settings.

In the manual setup you can setup smtp with username, server, port and encryption. The same as thunderbird, but in Firefox OS the option with the authentication method isn't available.

I tried to handle the authentication method with a xml-file. So now all options are set correct, including authentication method, but when I try to send an email I still get this error:

The page at app://email.gaiamobile.org says: Sending email failed