diff --git a/Dockerfile b/Dockerfile
index ac4a995..c8bd2ea 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -20,12 +20,23 @@ RUN --mount=type=cache,target=/usr/local/cargo/registry \
     fi && \
     cp target/release/tranquil-pds /tmp/tranquil-pds
 
-FROM alpine:3.23
-RUN apk add --no-cache msmtp ca-certificates && ln -sf /usr/bin/msmtp /usr/sbin/sendmail
+FROM alpine:3.23 AS signal-cli
+RUN apk add --no-cache curl tar
+ARG SIGNAL_CLI_VERSION=0.13.24
+RUN curl -fsSL "https://github.com/AsamK/signal-cli/releases/download/v${SIGNAL_CLI_VERSION}/signal-cli-${SIGNAL_CLI_VERSION}-Linux-native.tar.gz" \
+    | tar xz -C /usr/local/bin
+
+FROM debian:trixie-slim
+RUN apt-get update && apt-get install -y --no-install-recommends msmtp ca-certificates \
+    && rm -rf /var/lib/apt/lists/* \
+    && ln -sf /usr/bin/msmtp /usr/sbin/sendmail
+COPY --from=signal-cli /usr/local/bin/signal-cli /usr/local/bin/signal-cli
+VOLUME /var/lib/signal-cli
 COPY --from=builder /tmp/tranquil-pds /usr/local/bin/tranquil-pds
 COPY --from=frontend /app/dist /var/lib/tranquil-pds/frontend
 COPY migrations /app/migrations
 WORKDIR /app
+ENV SIGNAL_CLI_CONFIG=/var/lib/signal-cli
 ENV SERVER_HOST=0.0.0.0
 ENV SERVER_PORT=3000
 EXPOSE 3000
diff --git a/crates/tranquil-comms/src/sender.rs b/crates/tranquil-comms/src/sender.rs
index 0380a9f..84a2e5f 100644
--- a/crates/tranquil-comms/src/sender.rs
+++ b/crates/tranquil-comms/src/sender.rs
@@ -22,10 +22,13 @@ pub trait CommsSender: Send + Sync {
 
 #[derive(Debug, thiserror::Error)]
 pub enum SendError {
-    #[error("Failed to spawn sendmail process: {0}")]
-    ProcessSpawn(#[from] std::io::Error),
-    #[error("Sendmail exited with non-zero status: {0}")]
-    SendmailFailed(String),
+    #[error("Failed to spawn {command}: {source}")]
+    ProcessSpawn {
+        command: String,
+        source: std::io::Error,
+    },
+    #[error("{command} exited with non-zero status: {detail}")]
+    ProcessFailed { command: String, detail: String },
     #[error("Channel not configured: {0:?}")]
     NotConfigured(CommsChannel),
     #[error("External service error: {0}")]
@@ -160,14 +163,31 @@ impl CommsSender for EmailSender {
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .stderr(Stdio::piped())
-            .spawn()?;
+            .spawn()
+            .map_err(|e| SendError::ProcessSpawn {
+                command: self.sendmail_path.clone(),
+                source: e,
+            })?;
         if let Some(mut stdin) = child.stdin.take() {
-            stdin.write_all(email_content.as_bytes()).await?;
+            stdin.write_all(email_content.as_bytes()).await.map_err(|e| {
+                SendError::ProcessSpawn {
+                    command: self.sendmail_path.clone(),
+                    source: e,
+                }
+            })?;
         }
-        let output = child.wait_with_output().await?;
+        let output = child.wait_with_output().await.map_err(|e| {
+            SendError::ProcessSpawn {
+                command: self.sendmail_path.clone(),
+                source: e,
+            }
+        })?;
         if !output.status.success() {
             let stderr = String::from_utf8_lossy(&output.stderr);
-            return Err(SendError::SendmailFailed(stderr.to_string()));
+            return Err(SendError::ProcessFailed {
+                command: self.sendmail_path.clone(),
+                detail: stderr.to_string(),
+            });
         }
         Ok(())
     }
@@ -656,7 +676,10 @@ impl CommsSender for SignalSender {
                         retry_delay(attempt).await;
                         continue;
                     }
-                    return Err(SendError::ProcessSpawn(e));
+                    return Err(SendError::ProcessSpawn {
+                        command: self.signal_cli_path.clone(),
+                        source: e,
+                    });
                 }
                 Err(_) => {
                     if attempt < MAX_RETRIES - 1 {