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diff --git a/src/processing.rs b/src/processing.rs
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+// Copyright 2019 Alexandros Frantzis
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+//
+// SPDX-License-Identifier: MPL-2.0
+
+//! Email processing and filtering.
+
+use std::io::Write;
+use std::process::{Command, Output, Stdio};
+
+use crate::{Email, Result};
+
+impl Email {
+    /// Filters the contents of the email using an external command,
+    /// returning a new email with the filtered contents.
+    ///
+    /// The command is expected to be provided as a `&str` array, with the
+    /// first element being the command name and the remaining elements the
+    /// command arguments.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// use mda::Email;
+    /// let email = Email::from_stdin()?;
+    /// let email = email.filter(&["bogofilter", "-ep"])?;
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn filter(&self, cmd: &[&str]) -> Result<Email> {
+        Email::from_vec(self.process(cmd)?.stdout)
+    }
+
+    /// Process the contents of the email using an external command,
+    /// returning a `std::process::Output` for the executed command.
+    ///
+    /// The command is expected to be provided as a `&str` array, with the
+    /// first element being the command name and the remaining elements the
+    /// command arguments.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// use mda::Email;
+    /// let email = Email::from_stdin()?;
+    /// let output = email.process(&["bogofilter"])?;
+    /// if let Some(0) = output.status.code() {
+    ///     email.deliver_to_maildir("/my/spam/path")?;
+    /// }
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn process(&self, cmd: &[&str]) -> Result<Output> {
+        let mut child =
+            Command::new(cmd[0])
+                .args(&cmd[1..])
+                .stdin(Stdio::piped())
+                .stdout(Stdio::piped())
+                .spawn()?;
+
+        child.stdin
+            .as_mut()
+            .ok_or("Failed to write to stdin")?
+            .write_all(&self.data)?;
+
+        Ok(child.wait_with_output()?)
+    }
+
+    /// Creates an `Email` by filtering the contents from stdin.
+    ///
+    /// This can be more efficient than creating an `Email` from stdin and
+    /// filtering separately, since it can avoid an extra data copy.
+    ///
+    /// The command is expected to be provided as a `&str` array, with the
+    /// first element being the command name and the remaining elements the
+    /// command arguments.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// use mda::Email;
+    /// let email = Email::from_stdin_filtered(&["bogofilter", "-ep"])?;
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn from_stdin_filtered(cmd: &[&str]) -> Result<Self> {
+        let output =
+            Command::new(cmd[0])
+                .args(&cmd[1..])
+                .stdin(Stdio::inherit())
+                .output()?;
+
+        Email::from_vec(output.stdout)
+    }
+}