What DuoCircle LLC does

DuoCircle LLC is a privately held, self-funded email infrastructure provider founded in 2014 and headquartered in San Diego, California, operating as a remote-first organization with a distributed engineering team. The company began after acquiring Dyn's email hosting customer book in 2014 and has grown organically into a portfolio of specialized products organized into four groups: Authenticate (SPF Management via AutoSPF, DMARC Reporting as a multi-region G2 Category Leader, free Mail Flow Monitoring, and BIMI hosting), Protect (Phishing Protection, Spam Filtering, visual verification via Verisend for M365, and on-premises SpamSentinel/Inbox Genius for HCL Notes/Domino environments acquired from Maysoft in 2022), Deliver (Outbound SMTP relay operating since 2014, a Developer Email API at unsent.dev, Inbox Placement Testing via InboxIssue, and cold outreach NuReply with generative-AI personalization), and Route (Alumni Forwarding for universities, Tenant Migration, Email Forwarding, Backup MX, and per-message Message Replay).

The underlying platform is an email infrastructure stack covering authentication (SPF, DKIM, DMARC), protection (phishing, spam, BEC defense), delivery (SMTP relay, HTTP email API, inbox placement testing, cold outreach), and routing (forwarding, migration, backup MX). Notable proprietary features include automated SPF record flattening to address the 10-DNS-lookup limit, multi-region DMARC reporting, a no-password DKIM-validated smart host for M365 and Google Workspace, and bring-your-own-mailbox cold outreach architecture. Operational scale is material: approximately 2.1M SMTP messages relayed per day, 847K messages scanned for protection, 488K active forwarding rules, 12.4K SPF/DKIM/DMARC checks per day, and 99.99% Outbound SMTP delivery rate across 50,000+ organizations.

DuoCircle's revenue model is hybrid. Most products are sold on annual or monthly subscriptions (AutoSPF, DMARC Report, Phishing Protection, Spam Filtering, Backup MX, Alumni Forwarding, Tenant Migration, SpamSentinel/Verisend/Inbox Genius), while delivery products use usage-based pricing (Outbound SMTP from $4 to $550/month across tiers of 2,500 to 1,000,000 emails/month, with a 1,000-email free tier, and dedicated IP add-on at $45/month). Free tiers on Outbound SMTP, DMARC Report (first domain), and Mail Flow Monitoring drive self-serve product-led growth, while expert-led direct sales — with no SDR funnel and same-day response — serves enterprise customers. The company is profitable-implied and self-funded, with no disclosed venture capital or private equity investment. Brad Slavin serves as Manager and CEO/Founder.

Name

DuoCircle LLC

Legal name

DuoCircle LLC

Website

https://duocircle.com

Company type

Private

Founded year

2014

Operating status

Operating

Headcount range

11–50 employees

Short description

DuoCircle LLC is a self-funded email infrastructure provider offering authentication (SPF/DMARC), phishing protection, SMTP relay, and email routing products to 50,000+ organizations, serving SMBs via free-tier self-serve and enterprises through expert-led direct sales.

Ownership category

akta.pro rank

Product category

Email Security and Infrastructure

NAICS

Computer Systems Design and Related Services (5415), Computing Infrastructure Providers, Data Processing, Web Hosting, and Related Services (5182), Computing Infrastructure Providers, Data Processing, Web Hosting, and Related Services (518)

SIC

Services-Computer Programming, Data Processing, Etc. (7370)

akta.pro primary industry

Email Authentication & Domain Protection (DMARC/SPF/DKIM) (HDADAKAC)

Keywords

• Email security services
• SMTP relay services
• Email authentication tools
• Phishing protection software
• Transactional email delivery

Headquarters

HQ city

San Diego

HQ country

United States

HQ region

North America

Offices1 record

Markets served

GTM type

B2B

Offering type

Software

Cost components

Technology or R&D, Personnel, Infrastructure, Operations, Marketing or Sales

Revenue model

1. Email Authentication Products: Subscription and usage-based revenue from SPF management, DMARC reporting, and mail flow monitoring. DMARC Report covers first domain free; AutoSPF available as paid tier.
2. Email Protection Products: Subscription revenue from phishing protection, spam filtering, and Lotus Notes email security. Same SLA applied to all customers regardless of size.
3. Email Delivery Products: Usage-based pricing for SMTP relay (Outbound SMTP), Developer Email API (unsent.dev), Inbox Placement Testing (InboxIssue), and Cold Email Outreach (NuReply). Free tier of 1,000 emails/month for Outbound SMTP.
4. Email Routing Products: Subscription revenue from alumni forwarding, tenant migration, email forwarding, and backup MX services.
5. Installable Software: Per-user or per-domain license fees for SpamSentinel, Verisend, and Inbox Genius (HCL Notes/Domino products). Annual subscription with major-version upgrades included.

Pricing tiers

Go-to-market motion2 records

Distribution channels4 records

Marketing channels6 records

Core offering

DuoCircle sells a portfolio of specialized email infrastructure products across four functional groups: Authenticate (SPF Management via AutoSPF, DMARC Reporting, Mail Flow Monitoring), Protect (Phishing Protection, Spam Filtering, Verisend for M365, SpamSentinel for HCL Notes/Domino), Deliver (Outbound SMTP relay, Developer Email API at unsent.dev, Inbox Placement Testing via InboxIssue, Cold Email Outreach via NuReply), and Route (Alumni Forwarding, Tenant Migration, Email Forwarding, Backup MX). The portfolio serves 50,000+ organizations since 2014 with approximately 90% of category-leader capability at roughly half the price, backed by engineering-led expert support.

Product overview

DuoCircle LLC operates a portfolio of specialized email infrastructure products organized across four functional groups: Authenticate (SPF Management via AutoSPF, DMARC Reporting as multi-region G2 Category Leader, free Mail Flow Monitoring, and BIMI hosting), Protect (Phishing Protection, Spam Filtering, Verisend for M365 visual verification, and SpamSentinel/Inbox Genius for HCL Notes/Domino), Deliver (Outbound SMTP running since 2014, Developer Email API at unsent.dev, Inbox Placement Testing via InboxIssue, and Cold Email Outreach via NuReply with AI personalization), and Route (Alumni Forwarding, Tenant Migration for M365/Workspace, Email Forwarding, Backup MX, and Message Replay). The company serves 50,000+ organizations since 2014 with approximately 90% of category-leader capability at roughly half the price.

Differentiator

Problem solved

Functional benefit

Brands

• AutoSPF: SPF Management service that flattens SPF records to stay within the 10-DNS-lookup limit
• DMARC Report
• Mail Flow Monitoring
• Phishing Protection
• Spam Filtering
• Verisend
• SpamSentinel
• Outbound SMTP
• unsent.dev
• InboxIssue
• NuReply
• Alumni Forwarding
• Tenant Migration
• Backup MX
• Inbox Genius

Products and services

• AutoSPF SPF Management service that handles SPF record flattening to prevent organizations from exceeding the 10-DNS-lookup limit. Targets organizations running email on AWS or similar architectures who face SPF flattening failures that incumbent vendors will not address at their price tier.
• DMARC Report DMARC Reporting service that provides aggregate and forensic DMARC reports to help organizations understand and enforce email authentication. Multi-region G2 Category Leader for DMARC Software with a free tier for one domain and volume-based pricing for additional domains.
• Mail Flow Monitoring Free mail flow monitoring service providing visibility into email delivery and authentication status. Fully free since 2018 with no usage cap on monitoring.
• Phishing Protection Email security layer providing protection against phishing attacks, BEC defense, impersonation detection, and click-time URL inspection at roughly half the price of enterprise alternatives. Targets organizations needing protection against phishing, spam, and business email compromise attacks.
• Spam Filtering Spam filtering service that protects organizations from unwanted and malicious email as part of the DuoCircle protection portfolio.
• Verisend Visual mail verification for Microsoft 365 providing inbox-level visual verification for banking and financial services, helping end users verify email authenticity through visual cues.
• SpamSentinel On-premises spam filtering software for HCL Notes and Domino servers, originally published by Maysoft and now operated by DuoCircle with continuity of customer relationships. Includes open-source components ARSoft.Tools.Net, BouncyCastle, MimeKit, and Newtonsoft.Json.
• Inbox Genius Inbox-side filtering and triage product for HCL Notes and Domino environments, distributed under the same agreement as SpamSentinel. Uses libcurl (v7.68.0) for HTTP/HTTPS communication.
• Outbound SMTP SMTP relay service for application mail (transactional, system notifications, receipts, password resets) operating since 2014 with a 99.99% delivery rate. Free tier of 1,000 emails/month with no credit card required. Handles reputation management, throttling, and routing as the platform's responsibility.
• unsent.dev (Developer Email API) Developer Email API providing HTTP-first API access to the same delivery infrastructure as Outbound SMTP. Targeted at developers who prefer a clean REST API interface over configuring SMTP servers.
• InboxIssue Inbox Placement Testing service using real corporate-mailbox testing to measure and improve email deliverability to actual inbox destinations.
• NuReply Cold Email Outreach platform with bring-your-own-mailbox architecture connecting Google Workspace, Microsoft 365, or SMTP-capable accounts. Includes AI personalization per recipient from a base template, automated mailbox warm-up, and multi-step campaigns.
• Alumni Forwarding Email forwarding service designed for universities to handle alumni email forwarding and maintain communication with former students after graduation.
• Tenant Migration Migration service for Microsoft 365 and Google Workspace cutovers, helping organizations move between tenant environments with email routing support during transitions.
• Email Forwarding General email forwarding service for routing emails to desired destinations as part of DuoCircle's routing portfolio.
• Backup MX Backup MX service that holds mail when the primary mail server is unavailable, queuing and retrying delivery once the primary server is restored.

Quantifiable outcome

• About 90% of category-leader capability at roughly half the price
• +3 more outcomes

Named customers3 records

Segments7 records

Ideal customer profiles5 records

Technology focussed Yes

API detail

Has API

Yes

API docs

API detail

Core technology

AI maturity

App detail

Integration2 records

AI capability2 records

Feature8 records

Partnerships

Two partnerships are on record, tiered core and foundational.

• Maysoft (SpamSentinel, Verisend, Inbox Genius products)coreStrategic or Co-development Partner · 1 January 2022Maysoft, the email security specialist for IBM and HCL Notes/Domino, transferred their customer relationships to DuoCircle when their team was ready to step back. The reasoning was: 'You operate the way we do. We trust you to take care of our customers.' Same founding condition as Dyn: don't damage the brand, don't let customers down.
• Dyn (acquired email hosting customer book)foundationalStrategic or Co-development Partner · 1 January 2014In 2014, DuoCircle acquired Dyn's email hosting customer book. The condition was non-negotiable: don't damage their brand, don't let their customers down. This acquisition was the founding charter of the company and established their operating principle of taking care of every customer.

Scale indicators8 records

Recent moves6 records

Expansion highlights6 records

Direct peers

• Mimecast: Enterprise email security suite covering phishing, spam, and continuity. Targets the same mid-market and enterprise buyers as DuoCircle's Phishing Protection and Backup MX products, though at a substantially higher price point.
• Proofpoint: Largest pure-play enterprise email security vendor; directly named in DuoCircle's comparison pages. Phishing Protection, BEC defense, and DMARC/email authentication overlap heavily with DuoCircle's Protect and Authenticate portfolios.
• Smartlead: Explicitly named as a DuoCircle comparison target. Cold email outreach platform with mailbox warm-up, multi-step sequences, and AI personalization — directly overlapping with NuReply's feature set.
• Valimail: DMARC and SPF management specialist. Closest direct competitor to DuoCircle's AutoSPF and DMARC Report products in the email authentication category, with a similarly horizontal go-to-market.
• Postmark: Transactional email delivery service targeting application developers; overlaps with DuoCircle's Outbound SMTP and unsent.dev on reliable, reputation-managed transactional delivery.
• Twilio SendGrid: Named comparison target. Cloud SMTP relay and email API provider; directly competes with DuoCircle's Outbound SMTP and unsent.dev Developer Email API products in transactional and application email delivery.
• Instantly.ai: Fast-growing cold outreach and email warm-up platform. Competes head-to-head with NuReply on the sales-led cold-email use case, typically at a similar price point but with a stronger product-led growth motion.
• Mailgun: Email API and SMTP relay service from Sinch. Directly competes with Outbound SMTP and unsent.dev in the developer-focused transactional email category.
• EasyDMARC: Explicitly named as a DuoCircle comparison target. DMARC, SPF, DKIM, and BIMI services targeting the SMB-to-mid-market segment with a very similar product surface to DuoCircle's Authenticate group.

Broad incumbents

• Validity (ReturnPath): Email deliverability and sender reputation platform (formerly ReturnPath). Overlaps with DuoCircle's DMARC Report, Mail Flow Monitoring, and InboxIssue inbox-placement products for enterprise senders.

Market position

Strengths5 records

Weaknesses5 records

Competitive moat6 records

Key risks6 records

Key highlights6 records

Customer concentration

• Xx.com/duocirclellc
• Facebookfacebook.com/duocirclellc
• LinkedInlinkedin.com/company/duocircle

Compliance4 records

Revenue estimate

Valuation estimate

Number of profiles

Profiles1 record

Subsidiaries1 record

Funding overview

Funding rounds

Investors

Funding detail is available on the Subscription and Enterprise plan.Contact sales →

M&A

Investments

M&A and investment is available on the Subscription and Enterprise plan.Contact sales →

Frequently asked questions about DuoCircle LLC