This is a good question - as it demonstrates awareness of the potential impact of opening emails containing embedded images and linked content. Simply accessing an email can:
a) confirm the existence of an active email account to a potential Threat Actor
b) attempt loading of malware or other exploit
You can inhibit automatic download of embedded content. From iPad settings:
Settings > Mail > Load Remote Images - set to OFF
When Load Remote Images is disabled, opening a message in your Mail App will beneficially not automatically download linked content from embedded links. Providing that you do not tap Download All Images when accessing a mail message from he Mail App, remote content will not be downloaded when reviewing an email.
Whilst this safeguard prevents download and display of linked images and content, embedded images are displayed. It is presumed that this subtlety - and its significance - is understood. It follows that Mail attachments, if they exist, should not be accessed unless they are both expected and to originate from a trusted source.
As such, it should be possible to add malicious email messages to your Block List with minimal risk.
I hope this information and guidance is helpful to you.