All mail clients I know behave the same on default: Incoming mail is placed in a folder called “Inbox”, outgoing mail is stored in the “Sent” folder. Very intuitive, isn’t it? Not for me.
For me, mail is more like a conversation than a (s)he said/I said. Especially when a mail is sent to several recipients. Why would one remove the own statements from a discussion and keep all other statements together? For a very long time (actually ever since I started using mail), my mail clients are set up to save my sent mail in the Inbox.
At home I simply move away all mails of one month after two month, so the Inbox contains all mails of the current and the previous month. At work, I regularly sort mails in project folders, according to the project they belong to. Here it’s an advantage, too: My answers also belong to the project.
Things I would like to see in a mail client:
Remove the “Sent” folder. Or make it virtual. Conversations should be kept as one thread.
Add powerful virtual folders: One folder for all messages received from (and sent to) a specific person. (Automatically creating/removing these virtual folders for persons that appear/no longer appear in mails in the Inbox.)
Support a good quote style. This means: Help people to answer in-line instead of above the full quote. One huge step would be to show a placeholder at the top of the mail where the user types the greeting and, once he presses return, jump to the end of the first paragraph of the mail, insert a line-break and allow the user to answer that paragraph. Of course, typical greetings would be recognized and removed automatically from the quoted text.
The “didn’t-you-want-to-attach-something”-detection: If the mail contains phrases like “attached file” or “I’ll attach” but the mail has no file attached, then simply ask the user before sending to confirm this. Don’t forget to localize these phrases.