Yes, that df is what I was looking at in diagnostics, but I see I was fooled by the fact that it was 67% used because you have so little RAM.
Can you add more memory? I don't recommend trying to run dockers without at least 4G, and more is better.
Your little RAM is the reason you had to upgrade manually, and is possibly the reason your email is failing. You could try disabling the docker service, reboot, then see if the email will work.