We’ve made a minor change that I hope will avoid this issue
https://github.com/discourse/discourse/commit/c137aaceec6d8ab98a58daa6b0c21a9431b36fc2
We were using an attribute selector in CSS for these blockquote/code/pre styles, and email support for them are a bit patchy, so here we’ve switched to a element selector. If you continue to have issues after updating please let us know and we’ll do some more digging!