Multi-thread emails (Reply/Forward)
In short: don’t fake a reply or forward. Putting "Re:" or "Fwd:" on a campaign that was never part of a real conversation is a deceptive tactic that mailbox providers detect from the message headers, and it can violate anti-spam law. It erodes trust, invites spam complaints, and drags down your domain reputation. If you want recipients to recall an earlier message, reference it honestly instead.
Threading keeps a conversation consistent across many emails. You’ve seen it in subject lines like "Re: Fitness equipment order" or "Fwd: Fitness equipment order," sometimes stacked as "Fwd: Re: Fwd: Fitness equipment order."
The problem is that a genuine thread isn’t defined by the words in the subject line — it’s defined by hidden header data. Mailbox providers check that data, and conversation algorithms at Gmail, Outlook, and others specifically look for "Re:"/"Fwd:" subjects that have no matching thread history. When they find a mismatch, they treat it as a deceptive signal. If recipients notice there was never a prior conversation, many feel manipulated and mark the message as spam.
How threads are actually tracked
Email threading is defined by the message headers in the RFC 5322 standard, which are generated automatically when a message is sent:
- Message-ID — a unique code identifying a single message and its sender.
- In-Reply-To and References — values carried through a thread that link each reply back to the messages before it.
A receiving system recognizes a real chain by the presence (or absence) of In-Reply-To and References. You can type "Re:" or "Fwd:" into a subject line, and you can paste in quote breakpoints, divider lines, and salutations — but these headers cannot be forged. So a message with a "Re:" subject and no In-Reply-To/References doesn’t look like a genuine reply; it looks like a spoofed one.
The first message in a conversation has a Message-ID but no In-Reply-To or References:
Message-ID: <CAPvxx…_x2b4tg@mail.zylker.com>
Those two values appear only in genuine replies or forwards:
In-Reply-To: <CAPvxx…Qp2A@mail.zylker.com>
References: <CAPvxx…3EmCw@mail.zylker.com>
So a "Re:"/"Fwd:" subject with no matching thread headers is a weak, negative signal — not an automatic quarantine, but one that pushes you toward the spam folder, generates complaints, and, if a provider sees enough faked threads from one source, degrades that domain’s reputation. In severe cases the domain can be blocklisted. It can also run afoul of the law: the US CAN-SPAM Act prohibits deceptive subject lines, and CASL and GDPR expect subject lines to match the actual message.
What to do instead
- Don’t fake reply/forward threading — it’s the same pattern phishing uses, so filters are trained to catch it.
- Use a fresh, honest subject line for follow-ups. In tests, genuine subject lines reach the inbox at the same rate as first sends, while fake "Re:" threading lands in spam far more often.
- If you want recipients to recall an earlier campaign, reference it openly — in a quoted box or a short lead-in, as shown below.
Example 1 — reminding contacts of a previous campaign
When you send a campaign and want recipients to remember your earlier conversation:
Hello Charles,
This email is a continuation of our "BePro – Be a Hero" campaign from last month. We hope you’re eager to visit extraordinary places across the globe — here are the complete details for your trip.
This email is a continuation of our "BePro – Be a Hero" campaign from last month. We hope you’re eager to visit extraordinary places across the globe — here are the complete details for your trip.
Example 2 — following up after meeting in person
When you email someone you met at a trade show or conference and want to jog their memory:
Hi,
I’m reaching out following the conversation we had at the Hello Trade Show in Geneva. I hope you’re interested in our products, and I’d love for you to subscribe to our newsletters.
I’m reaching out following the conversation we had at the Hello Trade Show in Geneva. I hope you’re interested in our products, and I’d love for you to subscribe to our newsletters.
Both approaches jog memory honestly — without pretending an email thread exists that never did.
Conclusion
An honest strategy earns a better response than a fake one, and it protects the sender reputation your whole program depends on. Now that you know the consequences of faking multi-thread emails, build your campaigns genuinely.