Email signatures that consistently work.

Signaturmonster works as an SMTP proxy: all outgoing emails pass through the software, which automatically adds signature, CI branding and disclaimer. Signatures are maintained once centrally in the web UI — one change applies immediately to all senders, regardless of the email client.

Yes — Thunderbird is one of the best-supported clients. There is an official Thunderbird addon that synchronizes signatures directly from Signaturmonster and shows them live in the compose window. The SMTP proxy and the addon work together, so the signature is neither injected twice nor needs to be maintained manually.

Yes. The official Thunderbird addon (WebExtension, .xpi) is available for free. After installation it connects to your Signaturmonster instance and automatically shows the correct signature when opening the compose window — including variables, CI branding and disclaimer notice. Compatible from Thunderbird 91.

Yes. Since Signaturmonster works at SMTP level, Outlook, Thunderbird, Apple Mail and any other clients can be used simultaneously — each configures the SMTP proxy as the outgoing mail server. All automatically receive the correct signature, CI branding and disclaimer.

Yes. Since Signaturmonster works as an SMTP proxy, it is completely independent of the email client. Outlook is simply switched to use the proxy as the outgoing mail server. Outlook recipients automatically receive a static PNG as fallback for signatures with animated GIF banners (via MSO Conditional Comments).

Yes — by design. Signaturmonster runs completely on your own infrastructure. Your emails are processed exclusively on your own server and there is no connection to external SaaS services or cloud platforms for email delivery. When an email arrives at the SMTP proxy, it is briefly stored in a local queue (SQLite) on your server before being processed and forwarded. No permanent copy is created.

No — this is not a missing feature, but an architectural consequence. Signaturmonster modifies email content (signature, branding, disclaimer). A subsequent cryptographic signature could not claim that the sender authorized exactly this content. This applies to all content-modifying mail systems, including commercial cloud solutions like Exclaimer or CodeTwo. The key advantage over cloud solutions: Signaturmonster runs on your infrastructure — no third party ever touches your emails.

If you require personal end-to-end signatures (e.g. law firms, authorities with qualified electronic signature requirements): sign cryptographically in your email client before the email reaches the proxy, and disable all proxy functions via #sm:off.

Yes. Under System → Mail Queue in the admin interface, all entries are visible with status (pending / sent / failed), sender, recipient and subject. Individual emails can be downloaded there — useful for debugging, audit purposes or manual review of failed delivery attempts. Failed entries can be re-queued directly from the UI.

It depends on the specific requirements. For qualified electronic signatures (QES) under eIDAS, the cryptographic signature must be set personally in the email client before the email reaches the proxy. Using #sm:off disables all proxy functions for these emails. For standard business emails without QES requirements, Signaturmonster is fully suitable. Gateway signing at the organizational level (organization signature, not personal E2E) is planned for a future release.

Yes. Signaturmonster runs alongside Mailcow without any issues — both run as Docker containers. Email clients are redirected to the Signaturmonster SMTP proxy, which forwards the processed emails to Mailcow. The integration requires no changes to the Mailcow configuration.

Yes. The Free version of Signaturmonster is permanently free and fully functional — with one sender, Drag & Drop signature designer, rule-based assignment, variable system and Thunderbird addon. For teams with multiple senders, CI branding and GIF banners, there are paid tiers.

An SMTP proxy is software that intercepts outgoing emails before they reach the mail server. The email client sends the email to the proxy — the proxy adds signature, disclaimer and CI branding and forwards the finished email to the real mail server. The process is completely transparent to the sender, no plugin or client modification needed.

No. Nothing changes on the mail server. You only change the outgoing SMTP server in the email client to the IP address where Signaturmonster is running (port 587). Signaturmonster then forwards the emails to your real mail server.

Yes. Any system that sends outgoing emails via SMTP can be redirected to Signaturmonster — including JTL-Wawi, Lexware, Sage, DATEV and other ERP and accounting solutions. Configuration is done in the SMTP settings of the respective software.

A one-line installer sets everything up: Docker images, configuration, SMTP relay. For Linux and macOS: curl -fsSL https://raw.githubusercontent.com/choldermann/signaturmonster/main/install.sh | bash. For Windows there is a PowerShell script. Prerequisite is a running Docker installation.

You need a device running Docker — that can be a home server, a NAS (Synology, QNAP), a Proxmox LXC, a Raspberry Pi or a rented VM. Signaturmonster must be reachable on the local network so email clients can use the SMTP proxy.

Yes. You can configure any number of sender addresses and domains (depending on the license tier). For each address, individual signatures, CI profiles and disclaimer rules can be set up. Rule-based assignment ensures the correct signature is always applied.

No — Outlook does not support animated GIFs in emails. Signaturmonster automatically generates MSO Conditional Comments: Outlook recipients receive the static PNG from Scene 1 of the banner as a fallback. All other clients (Apple Mail, Gmail, Thunderbird etc.) see the animated GIF. Both variants are embedded via CID inline.

Yes. Apple Mail supports standard SMTP and can be redirected to Signaturmonster without any issues. Apple Mail recipients also see animated GIF banners in full, since Apple Mail — unlike Outlook — supports GIF animations.

Yes. Roundcube and other webmailers can point to Signaturmonster via their SMTP configuration. This means users of self-hosted webmail installations also benefit from centralized signatures and disclaimers.

Disclaimer texts are stored centrally in the web UI — as HTML and plaintext variants. Signaturmonster automatically appends them to every outgoing email that matches a configured rule. If a disclaimer changes (e.g. legal form, address), one change in the web UI is enough — all subsequent emails immediately carry the new text.

Yes. Wherever Docker runs, Signaturmonster runs. Tested on Raspberry Pi 4, Synology NAS (Container Manager), QNAP, Proxmox LXC (Debian/Ubuntu) and standard Linux servers. Resource requirements are low — a Raspberry Pi 4 with 4 GB RAM is sufficient.