What's a good subject for a follow-up email?
It depends on whether the follow-up continues a conversation or starts a new one. If nobody replied to your first email, reply to it: your email app keeps the subject and adds Re:, and the reader sees your first note underneath. Write a new subject only when something new has happened, such as a call, a meeting, a referral or a gap of months. The table sets out the choice; the braces are placeholders and every example is a sample.
| What happened | Thread | Subject line | Why |
|---|---|---|---|
| No reply to your cold email (follow-ups 1 and 2) | Same thread | Re: {your original subject} | The first email sits under the follow-up, so the reader has the context |
| Still no reply, and you have a new angle | Same thread, or a new one | {new angle} at {company} | A new subject in Gmail starts a new conversation, so only do it with a new reason |
| You had a discovery or sales call | New | {company} + {your company}: next steps | They will search for it later |
| You're sending a proposal or pricing | New | proposal for {company}: {scope} | Easy to forward to whoever signs |
| They missed the meeting | New | missed you at {time}: new times? | Says what happened and what you want |
| A colleague of theirs pointed you to them | New | {referrer's first name} pointed me to you | Use it only if it is true |
| You met at an event | New | met at {event}: your {topic} question | Names where they know you from |
| They asked you to check back in a named month | Same thread | Re: {original subject} | They asked; the history is the reason to read it |
| Months of silence, then a new trigger | New | after {company}'s {news} | The old thread is stale; the trigger is the reason |
For what to write under a same-thread subject, our guide to the cold email follow-up, with seven templates has the bodies and timing.
What do Gmail, Outlook and the email rules say about follow-up subject lines?
Changing the subject splits the thread in both big inboxes, and the message standard describes Re: as something a reply may add, once. Checked 4 November 2026:
- Gmail. Gmail Help on grouping emails into conversations says a conversation breaks off into a new conversation when the subject line changes, or when it gets to more than 100 emails.
- Outlook. Microsoft's Outlook help on conversation view says a conversation groups all the messages in the same thread that share a subject line. It also says messages are shown in conversations by default for most account types.
- The Re: prefix. RFC 5322, the internet message format, says a reply's subject may start with Re: followed by the original subject, and that only one Re: ought to be used, since more than one can lead to undesirable consequences. A subject reading Re: Re: Re: is a sign your tool is stacking them.
- US law. The FTC’s CAN-SPAM compliance guide says: "The subject line must accurately reflect the content of the message." It says the law makes no exception for business-to-business email, and each separate email in violation can bring penalties of up to $53,088. A Re: on a first email claims a conversation that never happened, which is hard to square with that rule. This is a summary, not legal advice.
- Gmail's sender rules. Google's email sender guidelines say not to send messages with subject lines starting with Re: or Fwd: unless they are actual replies or forwards.
- Outlook's bulk-sender rules. Microsoft's April 2025 post on requirements for high-volume senders lists using accurate subject lines and avoiding deceptive headers among its recommended practices.
What do subject line studies say about length and wording?
They disagree on length, because they measure different things. None of the three reports below, all vendor data (Hunter and Instantly sell outreach software; Belkins sells outreach services), breaks out follow-up subject lines on their own (checked 4 November 2026), so read them as evidence about subject lines in general, and remember that a same-thread follow-up carries the subject of your first email.
| Report | Data | Measured | What it found |
|---|---|---|---|
| Hunter, State of Email Outreach 2026 | 31 million emails sent by Hunter users in 2025 | Replies | Five-word subjects replied at 6.5%, against 4.8% for one word and 3.7% for nine. Quick question subjects replied at 2.5%, against 2.9% for others |
| Belkins, subject line study (2025) | Over 5.5 million emails from Belkins' own dataset, 1 January to 31 December 2024, plus data from Reply.io | Mostly opens | Two to four words had the highest open rate, 46%; one word 38%; ten words 34%. Personalised subjects had a 7% reply rate, against 3% without |
| Instantly, Cold Email Benchmark Report 2026 | Billions of cold email interactions across thousands of workspaces; the page says the data covers 1 January to 18 December 2025 | Share of replies | 58% of replies came on step 1 and 42% on later steps. Step 2 emails that feel like replies outperform formal follow-ups by about 30%, with no metric named |
Two things follow for a small team:
- Open-rate findings are the weakest. Belkins' length result is about opens, an unreliable measure; our post on whether open tracking hurts deliverability explains why. Hunter's is about replies, so weigh it more for length.
- Your first subject does most of the work. In Instantly's split, 42% of replies came after step 1, and if those steps stayed in the thread, they were read under the first email's subject. Write that one for the whole sequence, not just for day one.
One practitioner goes shorter than both datasets: our post on Jed Mahrle's cold email tips covers the agency founder's case for one to three words and for ranking the subject line last of nine variables.
Follow-up subject lines you can copy for a new thread
These are for follow-ups that start a new thread; same-thread follow-ups keep your original subject. Counting each placeholder as one word, all of them land in the three-to-six-word band where Hunter's reply rates were highest. Our page of 50 cold email subject lines has more for first emails and breakups; these are written not to repeat them.
| Situation | Subject line (sample) | Words |
|---|---|---|
| After a call | {company} + {your company}: next steps | 4 |
| After a call | your questions from {day} | 4 |
| Proposal | proposal for {company}: {scope} | 4 |
| Proposal | {company} pricing, as discussed | 4 |
| No-show | missed you at {time}: new times? | 6 |
| Event | met at {event}: your {topic} question | 6 |
| Event | after your {event} talk | 4 |
| Referral | {referrer's first name} pointed me to you | 5 |
| Asked to wait, thread lost | back in {month}, as you asked | 6 |
| New trigger | after {company}'s {news} | 3 |
| New trigger | {company}'s {new role} and {topic} | 4 |
| New angle | {topic} at {company}, another angle | 5 |
| Handover | {first name} asked me to follow up | 6 |
| Handover | taking over from {colleague} at {your company} | 6 |
Keep them lowercase and plain, and make the email deliver exactly what the subject promises. Use the referral and handover lines only when someone really did point you there or hand the account over.
How do you test a follow-up subject line on a small list?
Send two subjects to matched halves of the same list and count replies, not opens. Then check the gap is bigger than chance. A worked example with sample numbers:
- The test. Northwind Ltd (a sample company) sends version A to 200 people and version B to another 200. A gets 6 replies (6 ÷ 200 = 3%) and B gets 10 (10 ÷ 200 = 5%).
- The noise. At a reply rate around 4%, the spread you'd expect on 200 sends is about 2.8 replies (√(200 × 0.04 × 0.96) = 2.77), and for the difference between two such halves about 3.9 (2.77 × √2).
- The verdict. A gap of 4 replies is about one of those spreads (4 ÷ 3.9 = 1.0), which chance alone produces often. Keep going until each version has reached several hundred people, or treat the result as a hunch.
- For scale. Hunter's gap between five-word and nine-word subjects, applied to 200 sends each, is 13 replies against about 7 (200 × 6.5% = 13; 200 × 3.7% = 7.4). That gap of about 5.6 replies is still only about 1.3 spreads at that reply rate (√(200 × 0.051 × 0.949) × √2 = 4.4; 5.6 ÷ 4.4 = 1.3), so on 200 sends even Hunter's widest gap could be chance.
Before the test, run both lines through our free subject line tester. It starts each line at 100 and takes points off for length, risky phrases, capitals, exclamation marks, fake replies and emoji, and shows roughly how the line looks on a phone. It is a check for common problems, not a prediction of replies.
What do sales threads on Reddit say about threading follow-ups?
They split three ways, and nobody posts a test. We read five threads on whether to keep follow-ups in one thread, one in r/coldemail and four in r/sales, posted from November 2019 to November 2024, and read the 21 comments we could retrieve (one from a moderator bot), checked 4 November 2026. Treat this as what salespeople report:
- Mix the two. Commenters in four threads describe some steps in the first thread and some in new ones. In a November 2024 r/coldemail thread, one commenter wrote: "Run a thread for first two emails, use a new subject line for 3rd and 4th in a new thread". A January 2024 thread has one commenter starting over after three emails per thread, and a July 2023 thread has one person describing a HubSpot sequence that alternates.
- Stay in one thread. Commenters in three threads reply in the original thread; two say it helps the prospect see who they are or go back to the earlier emails. In the May 2020 thread, the highest-scored reply keeps the thread but edits the subject, which in Gmail would start a new conversation.
- New email each time. One commenter in a November 2019 thread writes a new email for each attempt.
- No data. The November 2024 poster asked for A/B tests; the only other reply said to run one.
Fix your follow-up subject lines this week
Mark each step same thread or new
Use the first table: follow-ups 1 and 2 stay in the thread, and anything after a call, meeting or long gap gets its own subject.
Send yourself the sequence
Check the replies show one Re:, not several, and that Gmail and Outlook keep them as one conversation.
Rewrite your first subject for the whole sequence
It will sit on top of every same-thread follow-up. Aim for a plain line that names their company or topic.
Fill the new-thread subjects from the copy list
Replace every placeholder and delete any line that isn't true for that person.
Test on replies
Two versions, matched halves, one change at a time. Read the result against the noise in the worked example.
For how many steps to run before you stop, our comparison of how many follow-up emails to send sets the 2026 reports from Hunter, Instantly and Belkins side by side.
Frequently asked questions
Is it follow up or follow-up in an email subject?
Both are correct, in different jobs. The Oxford Learner's Dictionaries list follow up, with no hyphen, as a phrasal verb, and follow-up, hyphenated, as a noun and an adjective. So write following up on our call when it is an action, and follow-up: our call or a follow-up question when it names the thing.
Should a follow-up email have Re: in the subject?
Only when it is a genuine reply to an email you sent. Your email app adds Re: when you reply, and the message standard says a single Re: is the expected form. Adding Re: to a first email pretends you have already spoken, and US law requires subject lines to reflect what the message contains.
What's a good subject line for a sales email?
A short, plain line about the reader rather than your product, such as their company and the problem you solve. In Hunter's 2026 report, built on emails its users sent in 2025, five-word subject lines had the highest reply rate. Test two versions on your own list and judge them by replies.
What is the best subject line for a cold email?
No single line wins everywhere, and the large reports disagree on length because some count opens and some count replies. Start with three to six words naming their company, role or a recent event, keep it lowercase, avoid hype, and keep the one that earns more replies across a few hundred sends.
Should I change the subject line on my third follow-up?
Only if the third email brings a genuinely new reason to talk. In Gmail, a changed subject starts a new conversation, so the reader loses your earlier emails above it. Salespeople in the Reddit threads we read do both, and none shared a test, so pick one approach and measure replies.
Sources
- Gmail Help: Group emails into conversations (checked 4 Nov 2026)
- Microsoft Support: View email messages by conversation in Outlook (checked 4 Nov 2026)
- RFC 5322: Internet Message Format (section 3.6.5) (October 2008; checked 4 Nov 2026)
- FTC: CAN-SPAM Act, A Compliance Guide for Business (checked 4 Nov 2026)
- Microsoft Defender for Office 365 Blog: Outlook's new requirements for high-volume senders (2 Apr 2025, updated 30 Apr 2025; checked 4 Nov 2026)
- Hunter: The State of Email Outreach 2026 (checked 4 Nov 2026)
- Belkins: B2B Cold Email Subject Lines and Engagement (2025 Study) (checked 4 Nov 2026)
- Instantly: Cold Email Benchmark Report 2026 (checked 4 Nov 2026)
- Oxford Learner's Dictionaries: follow up (phrasal verb) (checked 4 Nov 2026)
- r/coldemail: What's better: thread the same subject line or use a new one on every message? (posted 13 Nov 2024; checked 4 Nov 2026)
- r/sales: Outbounding - reply to previous email thread or start a new one? (posted 13 Jul 2023; checked 4 Nov 2026)
- r/sales: Email messages in same thread, or not? (posted 3 May 2020; checked 4 Nov 2026)
- r/sales: Emailing prospects a second time after no response: reply on first email thread or send a new message? (posted 7 Nov 2019; checked 4 Nov 2026)
- r/sales: Sales Sequence: threading or new thread? (posted 21 Jan 2024; checked 4 Nov 2026)
- Google Workspace Admin Help: Email sender guidelines (checked 4 Nov 2026)