You probably have a configuration problem related to wkhtmltopdf.
If on Windows, the unit conversion does not work, look at the Windows error message if you have one, this may indicate a potential lack of framework. (.NET, Visual Studio Redistributable, etc..)
If on Linux, the conversion does not happen with an error indicating an X server problem, you need to install the xvfb package and make the following commands in your linux (adapting them to your wkhtmltopdf installation)
mv /usr/bin/wkhtmltopdf /usr/bin/wkhtmltopdf_nohead cp wkhtmltopdf_xvfb /usr/bin/wkhtmltopdf chmod a+x /usr/bin/wkhtmltopdf
The contents of the wkhtmltopdf_xvfb file are as follows:
#!/bin/bash xvfb-run -a -s "-screen 0 640x480x16" /usr/bin/wkhtmltopdf_nohead "$@"
If the error is related to a network problem when retrieving the images from the mail, it is then up to you to decide whether you want to allow the rendition server to connect to the net to recover these images or to give wkhtmltopdf in the configuration of the rendition server a proxy that will resolve authorized URLs and block others (a proxy blocking all the images is possible but the rendering will be degraded).