The subject line is the first thing the recipient reads. It determines whether the email gets opened, when it gets opened, and how the recipient approaches it. It is the most important sentence in the email. Most people write it last, in thirty seconds, as an afterthought.
What a good subject line does
A good subject line tells the recipient what the email is about and what, if anything, they need to do. It is specific enough to be useful and short enough to be read in full in the inbox preview.
Good: "Decision needed: vendor selection by Friday"
Good: "Q3 budget approved — no action required"
Good: "Question about the March 15 deadline"
What a bad subject line does
A bad subject line is vague, generic, or misleading. It forces the recipient to open the email to find out what it is about, which is a small but real cost multiplied across every email they receive.
Bad: "Following up" (on what?)
Bad: "Quick question" (about what?)
Bad: "FYI" (about what, and why?)
Bad: "Hi" (this is not a subject line)
The action flag
If your email requires the recipient to do something, say so in the subject line. "Action required," "Decision needed," "Please review by [date]" — these flags tell the recipient immediately that the email is not just informational. They can prioritize accordingly.
The no-action flag
If your email is purely informational and requires no response, say that too. "FYI — no action needed" in the subject line is a gift. The recipient can read it at their convenience without wondering if they need to respond.
The RE: RE: RE: problem
Long email threads accumulate RE: prefixes until the subject line is "RE: RE: RE: RE: Quick question" and the thread has evolved into a completely different topic. When a thread changes subject, change the subject line. This is a small act of courtesy that makes the thread searchable and comprehensible.
The habit: Write the subject line after you write the email, not before. Once you know what the email says, you can write a subject line that accurately describes it.