There is no setting in preferences/emails to preclude the possibility of massive emails being sent.
Let’s say someone posts a reply to one of your posts, and that reply has a multi-megabyte screencast attached.
Well, it gets attached to the email too.
Other software might just include the text part of the reply, with links to the full reply, but Discourse throws in the full reply.
Yes, there is "email total attachment size limit kb" setting: what does it do exactly? but that is an admin tool. I’m talking about a way for users to avoid getting giant emails, without help from admins.
It seems like this is a case where thought has been given to restricting a problem from occurring, but it was not remembered to also allow end users to do the same about what affects them.
Remove images from emailed reply to forum is also only for admins.