You should be able to drag items from Zotero directly into an email. That won't work for some email clients, though; if you have issues, first drag the items into File Explorer (Windows/Linux) or Finder (macOS), then from there into an email.

If you use stored files (the default) and not linked files, then yes, you can delete the originals. Linked file attachments have a small "link" indicator on their icons, like this:

https://s3.amazonaws.com/zotero.org/images/forums/u6580876/1qp9i70hm0jin95ilhye.png
You can also right-click the attachment and choose Show File (Windows) or Show in Finder (macOS) to verify that it's been copied to your Zotero data directory.