One Unified Inbox for Instantly. Every reply answered in under 5 minutes.
Instantly sends across unlimited email accounts with warm-up and its deliverability network behind them. Underfive takes it from there. Every reply from every campaign and workspace lands in one Unified Inbox, where AI inbox agents write a human-like response, send it from the same mailbox, follow up, and book the meeting on your calendar.
- 50 free replies
- No credit card
- Two-minute webhook setup
Reply via Instantly 
Read & classified
Interested · drafted in 48s 
Instantly gets the reply. Your Unified Inbox wins the meeting.
Instantly already collects replies in its Unibox. The expensive gap is the hours between a prospect replying and someone on your team finding it, reading it, and writing back. Underfive closes that gap with one inbox across every workspace and agents that answer in minutes.
Reply time, around the clock
Every reply to an Instantly campaign gets a human-like response in under five minutes. Nights, weekends, and holidays included.
More likely to qualify the lead
Harvard Business Review found that firms contacting a lead within an hour were nearly seven times more likely to qualify it, meaning they reached a real decision maker, than firms that waited an hour longer (Oldroyd, McElheran and Elkington, HBR, March 2011). That window is the whole product.
Things to paste. That is the setup.
Your Instantly API key goes into Underfive. Your Underfive webhook URL goes into Instantly under Settings and Webhooks. Nothing in your campaigns changes.
Connected in three steps
One API key, one webhook URL. No changes to your campaigns.
Connect Instantly with your API key and webhook
Copy your Instantly API key and paste it into Underfive under Integrations. Underfive then gives you a webhook URL for that workspace. Paste it into Instantly under Settings and then Webhooks, and enable the Reply received event. Every Instantly workspace has its own API key, and every Underfive workspace has its own webhook URL, so connect them in pairs.
- One API key per Instantly workspace, one webhook URL per Underfive workspace
- Takes about two minutes, no code
- Campaigns, warm-up, and email account rotation are untouched
Integrations · Add tool Teach your inbox agents how you sell
Upload your offer, FAQs, pricing, and objection handling. Connect Google Calendar or Outlook. Then decide which reply categories send automatically and which wait for a human.
- Knowledge base and reply logic per workspace
- Auto-send or Require Approval, per category
- Calendar connected for direct booking
Replies are answered, followed up, and booked
When a prospect replies, Instantly sends the thread to your Unified Inbox. An inbox agent reads it, classifies it, writes a response in your voice, and sends it back through Instantly from the same email account. Interested prospects get a meeting on your calendar.
- Sent from the email account that received the reply, so deliverability stays with Instantly
- Smart follow-ups when a thread goes quiet
- Every conversation in one Unified Inbox across campaigns and clients
Unified Inbox- Meeting bookedJordan MeyerThursday at 10:30 works. Talk then.Meeting booked
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Replied · autoEmily TranCan you send over a case study first?Replied · auto -
Follow-up · Apr 2Omar HaddadWe're mid-contract until Q2, circle back then?Follow-up · Apr 2 -
Needs approvalLena SchulzWho else at your company should I loop in?Needs approval
Already on Instantly? You are five minutes away.
Sign up, swap the API key and webhook URL, and the first reply that lands in your Instantly workspace gets handled. Nothing changes in your sequences, email accounts, or warm-up.
What happens to each kind of reply
Your inbox agents classify every email reply before they act. You decide which categories auto-send and which wait for a human.
Interested
Answers, proposes times, and books the meeting on your calendar.
Pricing question
Replies with the pricing guidance from your knowledge base and moves toward a call.
Product question
Answers from your FAQs and offer details, in your tone of voice.
Objection
Handles the objection using your playbook and keeps the thread alive.
Not right now
Acknowledges, then schedules a follow-up for the timeframe they gave.
Referral
Thanks them and continues the conversation with the person they named.
Not interested
Closes politely and stops the thread. No more follow-ups.
Uncategorized
Goes to the approval queue so a human decides. Nothing is sent blind.
Categories and their behaviour are yours to edit per workspace. Nothing is sent without a category you approved.
One client per workspace. One inbox for all of them.
Instantly agencies already run a workspace per client. Underfive mirrors that structure on the reply side, so each client gets its own agents, knowledge base, calendar, and Slack channel, and every client's replies show up in one view.
Workspace pairs
Connect each Instantly client workspace to its own Underfive workspace with its own API key and webhook. Agents, knowledge, and approvals stay separate per client.
Workspace Agent
Turn it on for a workspace and the agent handles every positive reply in that workspace without trigger keywords. Built for one Instantly workspace per client.
Trigger keywords for shared workspaces
Running several offers through one Instantly workspace? Route replies to the right agent by keyword, such as a brand name or domain in the thread.
A workspace per client
Team members are unlimited on every plan and workspaces are unlimited on Agency, so adding a client never adds a per-seat fee on the reply side.
Two tools. One job each.
Instantly owns sending, warm-up, and deliverability. The Underfive Unified Inbox owns everything after the reply lands.
| Capability | Instantly | Underfive |
|---|---|---|
| Unlimited email accounts, warm-up, deliverability network | — | |
| Sequences, A/B tests, lead database, workspaces | — | |
| Replies collected in a Unibox per workspace | ||
| One Unified Inbox across every workspace and client | — | |
| Writes a human-like reply to each response | — | |
| Replies within 5 minutes, 24/7 | — | |
| Negotiates times and books the meeting | — | |
| Follows up when a thread goes quiet | — | |
| Approval queue with Slack notifications | — |
Watch the full setup
From a blank Underfive account to the first Instantly reply handled, in one short recording.
Instantly integration FAQ
Everything people ask before connecting.
Does Underfive set up the Instantly webhook for me?
Not yet. Instantly webhooks are added inside Instantly. Underfive shows you the webhook URL for your workspace, and you paste it into Instantly under Settings and then Webhooks and enable the Reply received event. It takes about two minutes.
Where do I find my Instantly API key?
In Instantly, open Settings and look for the API key section. Each Instantly workspace has its own key, so connect each workspace to Underfive separately if you run multiple clients.
Does Underfive reply from my Instantly email accounts?
Yes. Replies are sent through Instantly from the email account that received the reply, so the prospect sees one continuous thread from the same person. Your rotation, warm-up, and deliverability settings stay in Instantly.
Will this change my Instantly campaigns?
No. Campaigns, sequences, schedules, email accounts, and warm-up keep running exactly as they do today. Underfive only acts on replies.
Instantly already has a Unibox. What does Underfive add?
The Unibox collects replies inside one Instantly workspace. Underfive brings replies from every Instantly workspace and every other tool into one Unified Inbox, and its inbox agents actually write and send the response, follow up, and book the meeting.
Can I review replies before they go out?
Yes. Each reply category can be set to auto-send or require approval. Replies that need approval wait in your Underfive queue, and you can approve them from the app, from Slack, or through the Underfive MCP connector.
I run several clients on Instantly. How does that work?
Create one Underfive workspace per Instantly workspace and connect each with its own API key and webhook URL. Turn on the Workspace Agent so it handles every positive reply in that workspace. Team members are unlimited on every plan, workspaces are unlimited on Agency, and the Unified Inbox shows every client in one place.
What does it cost?
Start with 50 free replies and no credit card. After that, plans are priced by agent credits. See the pricing page for current plans.
Stop leaving Instantly replies unanswered.
Connect in under five minutes. Your first 50 replies are free, and nothing changes in your campaigns.
Not on Instantly yet? Learn about Instantly.