Turn off automatic download of attachments on iOS Mail
Currently there isn't  such a setting on iOS Mail app (as of iOS 12.0.1, Oct 2018).

A workaround that may help with detecting some unfiltered spam messages would be trying to preview the message before opening it. It could help increasing the preview size: go to Settings > Mail > Preview and increase it to 5 lines. 


* image from https://www.imore.com/how-to-manage-mail-settings-iphone-ipad