Your sequencer sends.
Underfive books the meetings.
Underfive is a Unified Inbox with AI inbox agents. Every cold email and LinkedIn reply lands in one place, gets a human-like answer in under 5 minutes, and ends with a meeting on your calendar.
Works with the tools you already send from
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Smartlead -
Instantly -
Email Bison -
PlusVibe -
Lemlist -
HeyReach -
Expandi -
GetSales.io -
Aimfox
What is Underfive?
Underfive is a Unified Inbox with AI inbox agents for cold email and LinkedIn outreach. It connects to the tools you already send from, gives every client its own workspace, answers every reply in under 5 minutes in your voice, follows up when a thread goes quiet, and books the meeting on your calendar. You keep your sequencer. Underfive handles everything after the reply.
One inbox, every tool
Email and LinkedIn replies from every sequencer land in one place. No cap on mailboxes.
A workspace for every client
Each client gets its own agents, knowledge, calendar and Slack channel. Switch in one click.
Answered in under 5 minutes
Every reply read and answered in your voice, day and night, in any language.
Booked on your calendar
Real times offered, the back-and-forth handled, the invite sent from your calendar.
Every reply, from every tool, in one inbox
Replies from Smartlead, Instantly, Email Bison, HeyReach and the rest land in one Unified Inbox, sorted by what the lead actually said. No more checking ten tabs. Clear drafts from the keyboard: press A to send and jump to the next one, Esc to undo within 5 seconds.
From "sounds good" to a meeting on your calendar
When a lead is interested, the agent offers real times from your calendar, handles the back-and-forth, and sends the invite. The thread is tagged Meeting Scheduled and your team gets a Slack alert. Round-robin spreads meetings across the team.
- Google Calendar and Outlook, or your Calendly or Cal.com link
- Round-robin across reps and calendars
- Spots meetings already booked so nobody double-books
Autopilot where you trust it. Approval where you don't.
Set each reply category to auto-send or require approval. Drafts that need a human wait in the Underfive app and in your Slack channel, where you approve, edit, or hand the thread to your client in one click. Nothing goes out blind.
Learn Approvals → Slack → Follow-ups →
How does Underfive work?
Connect your outbound tools, teach your inbox agents, and every interested reply in your Unified Inbox gets answered in minutes and booked on your calendar.
Connect your outbound tools
Paste an API key from your sending tool, or connect Gmail and Outlook. For most tools Underfive sets up the reply webhook for you, and every reply starts landing in one Unified Inbox.
- One API key per tool, no setup on most sending tools
- Email and LinkedIn replies in the same inbox
- Each client and campaign in its own workspace
PlusVibeEmail BisonSmartleadInstantlyLemlistHeyReachGetSales.io
GmailSet up your inbox agents
Point the agent at your website and docs, connect your calendar, and decide which replies send on their own and which wait for your approval.
- Knowledge learned from your site, pricing and FAQs
- Auto-send or Require Approval, per reply category
- Google Calendar or Outlook connected for booking
Connect calendarAnswered in minutes, booked on your calendar
When a lead replies, the inbox agent reads the whole thread, writes a human-like answer, sends it from the original mailbox or LinkedIn profile, and books the meeting. Quiet threads get a smart follow-up.
- Human-like replies in under 5 minutes, 24/7
- Meetings booked straight onto your calendar
- Smart follow-ups when a thread goes quiet
PlusVibeEmail BisonSmartleadInstantlyLemlistHeyReachGetSales.ioGmailWhy do fast replies book more meetings?
Companies that reply within an hour are nearly 7 times more likely to qualify a lead than those that wait longer. Underfive answers every reply in under 5 minutes, so interested leads are still interested when the times arrive.
Source: Harvard Business Review, "The Short Life of Online Sales Leads," 2011. Qualified means reaching a decision maker.
Trusted by agencies to fill their clients' calendars
Every reply answered in minutes, every interested lead offered a time on the calendar.
"You guys are really on the ball and have gotten the product to a point that it's fulfilling our original vision."
"Underfive has saved us a lot of time. It booked meetings directly in our clients' calendars, and our client was happy."
Erik PaulsonFounder, Vendisys · on Email Bison "Underfive has exceeded my expectations! I use SmartLead and the quality of replies that Underfive generates, combined with the inbox agent configuration, is exactly what I was looking for."
Suprava SabatFounder, AcquisitionX · on Smartlead Featured in Business.Scoop
Which tools does Underfive work with?
Keep the sequencer you already use. Underfive connects to your email and LinkedIn tools in a few minutes, replies from the original mailbox or profile, and books onto your calendar.
- Smartlead Email
- Instantly Email
- Email Bison Email
- PlusVibe Email
- Lemlist Email + LinkedIn
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Gmail Email -
Outlook Email - HeyReach LinkedIn
- Expandi LinkedIn
- Aimfox LinkedIn
- GetSales.io LinkedIn
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Google Calendar Calendar - Outlook Calendar Calendar
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Calendly Calendar -
Cal.com Calendar -
GoHighLevel Calendar & CRM -
HubSpot Calendar & CRM -
Slack Approvals and alerts -
Firecrawl Lead research - Request an integration Most ship within days
Questions, answered
What is Underfive?
Underfive is a Unified Inbox with AI inbox agents for cold email and LinkedIn outreach. It collects every reply from your sending tools in one place, answers each one in under 5 minutes, follows up, and books the meeting on your calendar.
How does Underfive book meetings?
When a lead is interested, the inbox agent proposes real times from your connected Google Calendar or Outlook calendar, handles the back-and-forth, and sends the invite from your calendar. The thread is tagged Meeting Scheduled. You can also share a Calendly or Cal.com link instead.
Does Underfive replace my sequencer?
No. Keep sending from Smartlead, Instantly, Email Bison, PlusVibe, Lemlist, HeyReach or your other tools. Underfive connects to them and takes over from the moment a lead replies, sending each answer from the original mailbox or LinkedIn profile.
Which tools does Underfive work with?
Underfive works with Smartlead, Instantly, Email Bison, PlusVibe, Lemlist, Gmail and Outlook for email, and HeyReach, Expandi, Aimfox and GetSales.io for LinkedIn. It books onto Google Calendar and Outlook, and connects to Slack and GoHighLevel.
Can I keep each client separate?
Yes. Every client gets its own workspace with its own inbox agents, knowledge base, calendar and Slack channel, and you switch between them in one click. Workspaces are included on every plan: 1 on Professional, 5 on Growth, 10 on Business and unlimited on Agency.
Can I approve replies before they are sent?
Yes. Each reply category can auto-send or require approval. Drafts that need approval wait in the Underfive app and in your Slack channel, where you can approve or edit them before they go out.
Can an AI inbox agent replace an SDR?
It replaces the repetitive part of SDR work: reading replies, answering common questions, following up and booking meetings, 24/7 and in under 5 minutes. Your team spends its time on the calls instead of the inbox.
How much does Underfive cost?
Plans start at $60 a month and every plan includes unlimited inbox agents, team members and connected mailboxes. You pay for the replies your agents handle, and you can start with 50 free credits and no credit card.
Reply playbooks, every week
How outbound teams answer, follow up and book from the replies they already get.
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10 min readYour next interested reply could be a booked meeting.
Connect your sending tool, teach your inbox agent, and let it book from the next reply. Setup takes about 5 minutes.
No credit card · unlimited agents, seats and mailboxes on every plan