diff --git a/Cargo.lock b/Cargo.lock
index de95138..24c5795 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -467,6 +467,17 @@ dependencies = [
  "slab",
 ]
 
+[[package]]
+name = "getrandom"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "300e883d756b2e4ec94e02791f39b04b522276138852cfc41d9fb7e904106099"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "r-efi",
+]
+
 [[package]]
 name = "glob"
 version = "0.3.3"
@@ -834,6 +845,12 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "r-efi"
+version = "6.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8dcc9c7d52a811697d2151c701e0d08956f92b0e24136cf4cf27b57a6a0d9bf"
+
 [[package]]
 name = "regex-automata"
 version = "0.4.14"
@@ -869,6 +886,7 @@ dependencies = [
  "sd-notify",
  "serde",
  "serde_json",
+ "tempfile",
  "tokio",
  "tokio-util",
  "toml",
@@ -1048,6 +1066,19 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "tempfile"
+version = "3.27.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32497e9a4c7b38532efcdebeef879707aa9f794296a4f0244f6f69e9bc8574bd"
+dependencies = [
+ "fastrand",
+ "getrandom",
+ "once_cell",
+ "rustix",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "terminal_size"
 version = "0.4.4"
diff --git a/Cargo.toml b/Cargo.toml
index 9e15930..cf3216f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -91,3 +91,4 @@ codegen-units = 1
 
 [dev-dependencies]
 indoc = "2.0.7"
+tempfile = "3.27.0"
diff --git a/src/server/fingerd/local_email.rs b/src/server/fingerd/local_email.rs
index 4c394a0..697839c 100644
--- a/src/server/fingerd/local_email.rs
+++ b/src/server/fingerd/local_email.rs
@@ -89,3 +89,120 @@ fn detect_new_mail_by_maildir(maildir_path: &Path) -> anyhow::Result<MailStatus>
         ))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use nix::{
+        fcntl::AT_FDCWD,
+        sys::{stat::UtimensatFlags, time::TimeSpec},
+    };
+    use std::{fs, os::unix::fs::MetadataExt};
+    use tempfile::TempDir;
+
+    fn set_file_times(path: &Path, atime: i64, mtime: i64) {
+        nix::sys::stat::utimensat(
+            AT_FDCWD,
+            path,
+            &TimeSpec::new(atime, 0),
+            &TimeSpec::new(mtime, 0),
+            UtimensatFlags::FollowSymlink,
+        )
+        .unwrap();
+    }
+
+    fn set_dir_mtime(path: &Path, mtime: i64) {
+        let metadata = fs::metadata(path).unwrap();
+        set_file_times(path, metadata.atime(), mtime);
+    }
+
+    #[test]
+    fn test_detect_new_mail_by_mailbox_new_mail_received() {
+        let tempdir = TempDir::new().unwrap();
+        let mailbox = tempdir.path().join("Mailbox");
+        fs::write(&mailbox, b"mail").unwrap();
+        set_file_times(&mailbox, 1_700_000_000, 1_700_000_100);
+
+        let status = detect_new_mail_by_mailbox(&mailbox).unwrap();
+
+        assert_eq!(
+            status,
+            MailStatus::NewMailReceived {
+                received_time: Utc.timestamp_opt(1_700_000_100, 0).single().unwrap(),
+                unread_since: Utc.timestamp_opt(1_700_000_000, 0).single().unwrap(),
+            }
+        );
+    }
+
+    #[test]
+    fn test_detect_new_mail_by_mailbox_mail_last_read() {
+        let tempdir = TempDir::new().unwrap();
+        let mailbox = tempdir.path().join("Mailbox");
+        fs::write(&mailbox, b"mail").unwrap();
+        set_file_times(&mailbox, 1_700_000_100, 1_700_000_000);
+
+        let status = detect_new_mail_by_mailbox(&mailbox).unwrap();
+
+        assert_eq!(
+            status,
+            MailStatus::MailLastRead(Utc.timestamp_opt(1_700_000_100, 0).single().unwrap())
+        );
+    }
+
+    #[test]
+    fn test_detect_new_mail_by_maildir_no_mail() {
+        let tempdir = TempDir::new().unwrap();
+        let maildir = tempdir.path().join("Maildir");
+        let new_dir = maildir.join("new");
+        let cur_dir = maildir.join("cur");
+        fs::create_dir_all(&new_dir).unwrap();
+        fs::create_dir_all(&cur_dir).unwrap();
+
+        let status = detect_new_mail_by_maildir(&maildir).unwrap();
+
+        assert_eq!(status, MailStatus::NoMail);
+    }
+
+    #[test]
+    fn test_detect_new_mail_by_maildir_new_mail_received() {
+        let tempdir = TempDir::new().unwrap();
+        let maildir = tempdir.path().join("Maildir");
+        let new_dir = maildir.join("new");
+        let cur_dir = maildir.join("cur");
+        fs::create_dir_all(&new_dir).unwrap();
+        fs::create_dir_all(&cur_dir).unwrap();
+        fs::write(new_dir.join("msg"), b"mail").unwrap();
+        set_dir_mtime(&cur_dir, 1_700_000_000);
+        set_dir_mtime(&new_dir, 1_700_000_100);
+
+        let status = detect_new_mail_by_maildir(&maildir).unwrap();
+
+        assert_eq!(
+            status,
+            MailStatus::NewMailReceived {
+                received_time: Utc.timestamp_opt(1_700_000_100, 0).single().unwrap(),
+                unread_since: Utc.timestamp_opt(1_700_000_000, 0).single().unwrap(),
+            }
+        );
+    }
+
+    #[test]
+    fn test_detect_new_mail_by_maildir_mail_last_read() {
+        let tempdir = TempDir::new().unwrap();
+        let maildir = tempdir.path().join("Maildir");
+        let new_dir = maildir.join("new");
+        let cur_dir = maildir.join("cur");
+        fs::create_dir_all(&new_dir).unwrap();
+        fs::create_dir_all(&cur_dir).unwrap();
+        fs::write(cur_dir.join("msg"), b"mail").unwrap();
+        set_dir_mtime(&new_dir, 1_700_000_000);
+        set_dir_mtime(&cur_dir, 1_700_000_100);
+
+        let status = detect_new_mail_by_maildir(&maildir).unwrap();
+
+        assert_eq!(
+            status,
+            MailStatus::MailLastRead(Utc.timestamp_opt(1_700_000_100, 0).single().unwrap())
+        );
+    }
+}