On firefox 45, remote images are not shown in the webmail because of the CSP:

```
Content Security Policy: The page's settings blocked the loading of a resource at https://sendy.nitrokey.com/uploads/1431348652.png ("img-src https://mail.fripost.org").
```

Oh wait, that's weird: it seems to block data-urls too:

```
Content Security Policy: The page's settings blocked the loading of a resource at data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw== ("img-src https://mail.fripost.org").
```