SMTP Family Tree – 334digital.com

One forest. Solid lines are real code ancestry or product continuation. Dashed ideas are design influence or “speaks SMTP to,” not shared source.

Internet mail / SMTP
│
├─ 1970s Unix MTAs (contemporaries, not forks)
│  │
│  ├─ delivermail (Allman, Berkeley, 1979)
│  │    └─ Sendmail (1983 → Sendmail 8 → Proofpoint)
│  │         ├─ IDA / KJS  (historical variants)
│  │         └─ MeTA1 / “Sendmail X”     [NEW code; qmail/Postfix-inspired]
│  │
│  ├─ MMDF (Crocker, Delaware, ~1978) → MMDF II
│  │    └─ PMDF (Pascal → C)
│  │         └─ Sun Java System Messaging Server
│  │              └─ Oracle Communications Messaging Server
│  │
│  └─ Smail-1 (UUCP) → Smail-2 (name mostly)
│       └─ Smail-3
│            └─ Exim (Hazel, 1995)   [rewrite of Smail-3 ideas, new code]
│                 Debian/Ubuntu default
│
├─ 1990s clean-slate Unix MTAs
│  │
│  ├─ qmail (Bernstein, 1995)
│  │    ├─ netqmail → notqmail
│  │    ├─ s/qmail, indimail-mta, qmail-sagredo
│  │    └─ qpsmtpd (Perl front-end)
│  │         Maildir, CDB, QMQP spread from here
│  │
│  ├─ Postfix (Venema, 1997)
│  │    └─ embedded as SMTP engine in Zimbra
│  │
│  ├─ Courier-MTA
│  ├─ ZMailer
│  └─ later: OpenSMTPD (OpenBSD), dma (DragonFly)
│
├─ Groupware / store-centric  (SMTP at the edge)
│  │
│  ├─ Microsoft
│  │    XENIX mail (internal) ──x── no code link
│  │    Microsoft Mail / X.400 Exchange 4–5.5
│  │         └─ Exchange 2000+ (SMTP is the backbone)
│  │              ├─ Exchange Server (on-prem)
│  │              └─ Exchange Online + EOP
│  │                   smtp.office365.com
│  │
│  ├─ Lotus Notes → HCL Domino
│  └─ Zimbra = Postfix MTA + Java mailbox (wrap, not fork)
│
├─ Appliance / commercial gateways  (filter + relay, own engines)
│  │
│  ├─ IronPort (2000)
│  │    FreeBSD → AsyncOS (C + Python shrapnel)
│  │         A-series (outbound) / C-series ESA
│  │         Cisco buy 2007 → brand gone ~2012
│  │              ├─ Cisco Secure Email Gateway (ESA)
│  │              └─ Cisco CES / iphmx.com (hosted AsyncOS)
│  │
│  └─ same product class, other code:
│       PowerMTA, Halon, Barracuda, Proofpoint (also owns Sendmail Inc.)
│
└─ Cloud SMTP  (planetary scale or paid smarthost)
     │
     ├─ Mailbox clouds
     │    ├─ Gmail / Workspace     smtp.gmail.com, smtp-relay.gmail.com
     │    │    Google in-house MTA — not Postfix/Sendmail
     │    ├─ Exchange Online       (see Microsoft branch above)
     │    └─ Yahoo, Zoho, iCloud, Fastmail   (custom / mixed)
     │
     └─ Send-only relays  (your Postfix/Exim becomes the client)
          Amazon SES
          SendGrid (Twilio)
          Mailgun (Sinch)
          Postmark
          SparkPost → Bird
          Mandrill / Mailchimp Transactional
          Brevo, Resend, SMTP2GO, Mailjet…

There is almost no shared source-code tree among the big SMTP servers. Postfix is not a Sendmail fork, Exim is not a Sendmail fork, Exchange shares no Unix MTA code. What exists is a small number of real code lines, plus a larger web of design influence and compatibility (especially the sendmail command-line interface).

How to read the lines

Line	Meaning
`└─` parent–child	Same project, fork, rewrite-in-place, or corporate continuation
hanging list under a node	Siblings, not descendants
“inspired / wrap / speaks”	Design or protocol only

Almost nothing in the bottom two thirds shares C with Sendmail. The live public-MX majority is still Exim + Postfix; the volume majority is Gmail + Exchange Online + the send-only clouds.

One hop that ties the whole map together

app
 → local Postfix / Exim / Sendmail     (Unix branch)
   → SES or SendGrid or smtp.office365.com or CES
     → Gmail / EOP / another Postfix / ESA

That is the modern tree in motion: classic MTA as a stub, cloud or appliance as the real sender.

Three tiers: Type (job in the mail path) → Vendor (who owns the engine) → Instance (the thing you actually install or buy). Product name and 2026 roadmap sit on the instance.

TYPE
 └─ VENDOR
      └─ INSTANCE   product name · note / roadmap

1. Self-hosted Unix MTA

Full SMTP queue on a machine you run. Local delivery optional.

Vendor	Instance	Product name	Note / roadmap
Proofpoint (Sendmail Consortium)	Sendmail 8	Sendmail Open Source / Sentrion (commercial)	OSS still patched (~8.18 / 8.16.1 line). Not a growth product; enterprise leftover under Sentrion.
Proofpoint	MeTA1	MeTA1 (ex–Sendmail X)	Separate codebase. Quiet; not a Sendmail 8 upgrade path.
Independent (Venema)	Postfix	Postfix 3.11 (current), 3.10–3.8 still supported	Default on RHEL-family. Steady C releases; no “cloud rewrite.”
University of Cambridge / Exim Maintainers	Exim	Exim 4	Debian/Ubuntu and cPanel default. Monolithic; routing language is the product.
DJB / community	qmail family	qmail 1.03 → netqmail → notqmail 1.09	Frozen upstream; notqmail is the living fork.
OpenBSD	OpenSMTPD	OpenSMTPD (portable + base)	Default on OpenBSD. Small, privilege-separated.
Sam Varshavchik	Courier	Courier-MTA	Integrated ESMTP+IMAP. Niche.
DragonFly	dma	DragonFly Mail Agent	Leaf / outbound stub only.

Roadmap for this type: stay on-box or become a smarthost client of type 5. New greenfield boxes pick Postfix or OpenSMTPD, not Sendmail.

2. Groupware / mailbox platform

Store + calendar + SMTP. SMTP is a component, not the product.

Vendor	Instance	Product name	Note / roadmap
Microsoft	On-prem Exchange	Exchange Server Subscription Edition (SE)	Replaced 2016/2019 (EoS Oct 2025; ESU winding down). SE follows Modern Lifecycle (support while current; floor discussed ~2035). CU1 2026 adds features; CU2 ends coexistence with 2016/2019. Hybrid must move off EWS → Graph (EWS default-off Oct 2026, gone Apr 2027).
Microsoft	Cloud mailbox	Exchange Online + EOP	Same family, multi-tenant. smtp.office365.com. EWS retirement same window. High-volume sending being squeezed toward dedicated SKUs.
HCL	Domino	HCL Domino	Notes lineage. Independent of Unix MTAs.
Synacor / Zimbra	Zimbra	Zimbra Collaboration	MTA instance is Postfix (type 1 embedded).
Oracle	Messaging Server	Oracle Communications Messaging Server	PMDF → Sun → Oracle line. Legacy enterprise.

3. Security gateway / appliance

Sits on MX or outbound edge. Relays to type 1 or 2. Does not own mailboxes.

Vendor	Instance	Product name	Note / roadmap
Cisco (ex-IronPort)	On-prem appliance	Cisco Secure Email Gateway (ESA / SEG)	AsyncOS 16.5 GD Aug 2026; 16.0 maintenance ending. 12-month software maintenance after GD. New buys pushed off-box. CVE-2025-20393 (Spam Quarantine) is active-exploit context — patch or isolate the UI.
Cisco	Hosted gateway	Cisco Secure Email Cloud Gateway (CES)	Strategic instance. MX → `iphmx.com`. ESA→CES is the published migration. ETD is the M365 API add-on, not an MTA.
Cisco	Manager	Secure Email and Web Manager (SEWM / SMA)	Policy/reporting plane for ESA/CES, not an MTA.
Proofpoint	Cloud / appliance	Proofpoint Email Protection / Essentials	Competitor class to CES. Also owns Sendmail Inc.
Broadcom / others	Appliance	Symantec / Barracuda / Fortimail / Mimecast	Same type, other code.

4. Cloud mailbox

Public MX + IMAP/web. You do not run the MTA.

Vendor	Instance	Product name	Note / roadmap
Google	Consumer / Workspace	Gmail · smtp.gmail.com · smtp-relay.gmail.com	In-house MTA. App passwords / OAuth; “less secure apps” dead. Relay for printers/apps. MTA-STS first-class.
Microsoft	see type 2	Exchange Online	Listed there; also a type-4 mailbox cloud.
Yahoo, Zoho, Apple, Fastmail	respective mail	Yahoo Mail, Zoho Mail, iCloud Mail, Fastmail	Custom stacks. Fastmail historically mixed open components + in-house.

5. Cloud send-only / ESP

You inject (SMTP or API). They deliver. No mailbox.

Vendor	Instance	Product name	Note / roadmap
Amazon	SES	Amazon Simple Email Service	email-smtp.region.amazonaws.com. Cheapest raw pipe ($0.10/1k). You bring Postfix/Sendmail as the client.
Twilio	SendGrid	Twilio SendGrid	smtp.sendgrid.net. Brand folding into Twilio; permanent free tier gone (trial only).
Sinch	Mailgun	Mailgun	smtp.mailgun.org / .eu. API + inbound parse.
ActiveCampaign	Postmark	Postmark	smtp.postmarkapp.com. Transactional-only culture.
Bird	SparkPost	Bird Email (ex-SparkPost)	Enterprise volume; branding after MessageBird.
Intuit / Mailchimp	Mandrill	Mailchimp Transactional	Requires paid Mailchimp.
Independents	various	Brevo, Resend, SMTP2GO, Mailjet, MailerSend, Elastic Email	Same type; pick on price vs DX vs inbox tests.

How the three tiers compose

Type 4/2 mailbox cloud     Type 3 gateway        Type 5 ESP
 Gmail / EXO / CES MX   ←── ESA / CES / Proofpoint
                                 ↑
                    Type 1 Postfix / Exim / Sendmail
                    (or Zimbra’s embedded Postfix)
                                 ↑
                              app / MSA

Roadmap in one line: type 1 shrinks to a local stub; type 3 on-prem shrinks toward type 3/5 cloud (CES, Proofpoint, SES); type 2 on-prem is SE-or-migrate; type 4/5 take the public internet.