iOS e-mail app without automatic downloading of large e-mails
I think you can't stop the whole email downloading.

This is because an email is a stream of text the format is defined in this RFC. The programs that transmit and receive the email do not look at what the data is in the body of the email and so have to send all or nothing.

The files downloaded from other servers are represented in the email body as just text containing the name of the external file (a URL).

The mail client (the app on your iPhone) is the only software that looks at the body and it recognises certain plain text that the text after it is an encoded image.

Apple's mail clients including mail on the iPhone allows the sender to decide if large attachments are not put directly in the email body but held in iCloud as a external file so take up the small space as other external linkss.. Apple email attachment document.

There is a reason to use text only email :)