OpenSMTPD

opensmtpd.org · checked 7 Oct 2026

#23 · Mail Server Software

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How it plugs in

—Public API

NoOpen source

YesFree tier

Signal on Mail Server Software5.9#23 of 34

OpenSMTPD implements server-side SMTP and lets machines exchange email with systems that speak SMTP.

Pricing

OpenSMTPDFREE · Server-side SMTP implementation; runs on supported Unix systemsFree

As opensmtpd.org lists them, checked 7 Oct 2026.

Near it

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Runs on

Web
Windows
Mac
Linux
iOS
Android

Facts

Website: opensmtpd.org
Price: Free tier
Free trial: No
Delivery: Supported delivery methods include LMTP, Maildir, mbox, mail delivery agent commands, and relaying to another SMTP server.
Filtering: Mail filters can evaluate SMTP sessions at phases including connection, HELO/EHLO, sender, recipient, data, and commit.
License: The project says the software is freely usable and reusable under an ISC license.
Mail handling: It accepts matching messages into a mail queue and dispatches them according to configured actions.
Packaging: The portability team adds code for other operating systems but does not package OpenSMTPD for them.
Queue encryption: The configuration supports encrypting queue files with AES-256-GCM.
Security: Project goals include careful validity checks on network input and privilege separation to mitigate possible security bugs.
Security reporting: The project directs security issue reports to its private developers mailing list.
Support: The project is volunteer-developed and is not backed by a commercial entity.
TLS: Configuration supports STARTTLS and SMTPS for incoming connections, and TLS options for relaying mail.

On these boards

Mail Server Software#23 of 34

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