It is usually a good practice to use descriptive Subjects in your emails (i.e. mention your company name or purpose of email). If you are forwarding an email with a vague Subject line to someone else, it is perfectly acceptable to append/edit the original Subject before sending.
If you are an organization fanatic and use Outlook as your email client, you can easily edit the Subject of a received email in your Inbox for better organization.
Changing the (E-Mail) Subject
But what if you use the Mail application on your Apple Macintosh? The Apple Mail app will not let you do this. So to work around this, simply drag the incorrectly titled email or vaguely subjected message out of your Inbox into your Drafts folder. Next double click and edit the message by changing the Subject line, and then click Save. Drag the message back to your Inbox, and you’re finished! It should be noted that the received date and time of the message will change when you do this.
What if you don’t see a Drafts folder, you say? If you do not currently have any saved Drafts, the Mail apps will hide the Drafts folder. Simply create a temporary Draft message and either delete the Draft message when you are done or keep it saved indefinitely to always have the Drafts folder visible and ready to go if the need should arise when you need to rename the Subject of a previously received email in your Apple Mail program.