Better manage vertical & horizontal space in message list
Each primary message in the message list requires too much vertical and horizontal space for the information provided. By moving the location of the date and reply/action arrow inward and upward, you could collapse the vertical space and make this appear less "chunky".
Consider the circular name/profile icons to the left of each message: why should the icon take up 2/3 of the vertical height of the message? It should ideally cover the full vertical height of the message or else map to a single line.
Attachments and picture previews adding to the vertical height seems fine, but feels like a "condensed" view that takes messages to a single row height (as they look when expanded in a conversation thread) would be worth some mock-ups and user testing.
Horizontal space is less problematic but user control over the column width is limited - it only condenses so far. The location of the date also limits the available width to show the subject line.
This may seem minor but the usefulness of the message list column is for user context, orientation, and message to message selections while viewing the selected thread simultaneously on the right. Be careful about mixing purposes between this message list column and the message / thread view column.