I have "Load Remote Images" turned off on my phone and "Load remote content in messages" turned off on my mac.
I got a suspicious message on my phone, but wanted to check it out because not many make it through my filter these days. I noticed that there was a .pdf attachment and that it was immediately downloaded (not ideal). The message was sent to a gmail account I was accessing through IMAP.
Then when I went to my mac to mark the message as junk (I can't remember why I didn't mark it on my phone) the .pdf showed up inline and it was definitely spam. At this point I was irritated and then remembered that my phone had downloaded the .pdf when I opened the message before.*
What setting keeps my phone from downloading attachments automatically? I know it would be better to not open messages that might be spam, but sometimes I can't tell from just the subject.
*I'm assuming because it was downloaded on my phone, the mac Mail decided it was ok to download, or possibly accessed a version that the phone downloaded. Otherwise I need to ask a separate question about how to keep macOS Mail from downloading attachments.
Best Answer
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