The inbox may keep track of mails you receive, but would you like to also keep track of the ones you send? Then we have just stumbled upon an addon that might interest you: SentMail.
This mod does something simple, yet quite unique: it adds an "outbox" tab to your mailbox. In this tab you can see the latest mails you have sent from your current character. Every single detail of the mail can be read, including sent items and collect on delivery demands.
It is quite a wonder how this feature was not implemented in the game on the first place. Not even the most bulky tracking addons, such as Armory, keep track of outgoing mail, only focusing on the mail you receive. But SentMail is here to pick up the slack, not only providing the outbox but also making it look like it is part of the game itself.
Keep in mind that SentMail only records mail sent from the same computer, while the addon is enabled. For instance, it cannot be used to see mail sent by account hackers.