To: and Cc: should be stored as arrays

To and Cc headers are currently stored as a raw string with embedded new-line characters.
Would it not be better to store them as arrays of individual addresses?

Start by locating where To and Cc headers are parsed and stored, then inspect any existing tests covering header values. Confirm how embedded newlines are handled and identify the expected representation for individual addresses. Done means both headers are consistently stored as arrays without breaking existing parsing or retrieval behavior.

Written by the indexing model from the issue text.