Hi,
Reporting a possible bug on swift we found.
The template for images on emails - Templates/Designs/Swift/Swift_Email/Components/Image.cshtml is not including the host name in the image element. Here is the fix we applied.
string image = $"{Dynamicweb.Context.Current.Request.Url.Scheme}://{Dynamicweb.Context.Current.Request.Url.Host}/Admin/Public/GetImage.ashx?width={imageWidth}";