If the copy-pasted email is just plain text, and if it is not critical that it renders specifically as a block quote, you can just put it in a code block:
```
line1
line2
line
````
which renders here on Discourse as:
line1
line2
line
You can even add “quote” to the opening triple backticks if you want.
Also, per the CommonMark spec, whether line breaks are preserved in non-code blocks is up to the renderer. For example, in GitHub comments, which are GFM-based and thus a CommonMark superset, line breaks are preserved, including in block quotes. Many CommonMark tools and renderers have this as a user option, e.g. CommonMark.js (see it’s softbreak option), Markdown-it, Typora, etc.
You can also force a line break with \ or two spaces at the and of a line.
Finally, please note the announcement at the top of this forum. If you’ve already dismissed it, log out (and maybe clear cookies) or open the forum in a private browser mode window to have it appear again.