Show Sender's Address In Preview Pane
In the inbox, show the sender's email address if the sender is not in Contacts. So much mail is unsolicited from sales people. With Entourage, you can see the sender's address. This allows the recipient to see the part after the @ such as abcxyz.com, then check out that company before opening unsolicited mail. Otherwise, the recipient must open unsolicited mail to find out what it's about. At that point, a pixel is sent back to the sender, telling them they have a live, possibly interested fish on the line, which results in a continuous flow of mail from that sender. Entourage was awesome. Allowing recipients to see sender's address before opening was 1 reason why.
Marc Uecker commented
In the message list (and when reading an email), instead of only showing the display name (which can be modified by phishing and scammers), please add an option to show the sender's address (either behind like "Microsoft.com (firstname.lastname@example.org)" or only see the sender's email like "email@example.com".
Showing only the display name is a huge security flaw, and is probably responsible for 80-90% of successful phishing attacks.
Additionally, on the mobile web version, there is right now no way to even see that address because it's implemented via mouseover (not available on touch devices).
This is probably one of the most important pieces of information about an email message - who it's from (important: who it's from, not who that person claims to be). And you simply decide to not make that information available at all to the users. This is unacceptable. Please fix it.