diff --git a/Cargo.lock b/Cargo.lock
index 4313294..4e94149 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -577,6 +577,15 @@ dependencies = [
  "miniz_oxide",
 ]
 
+[[package]]
+name = "float-cmp"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "98de4bbd547a563b716d8dfa9aad1cb19bfab00f4fa09a6a4ed21dbcf44ce9c4"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "fnv"
 version = "1.0.7"
@@ -1273,6 +1282,12 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "normalize-line-endings"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61807f77802ff30975e01f4f071c8ba10c022052f98b3294119f3e615d13e5be"
+
 [[package]]
 name = "nu-ansi-term"
 version = "0.46.0"
@@ -1382,7 +1397,10 @@ checksum = "7e9086cc7640c29a356d1a29fd134380bee9d8f79a17410aa76e7ad295f42c97"
 dependencies = [
  "anstyle",
  "difflib",
+ "float-cmp",
+ "normalize-line-endings",
  "predicates-core",
+ "regex",
 ]
 
 [[package]]
@@ -1927,6 +1945,7 @@ dependencies = [
  "insta",
  "itertools",
  "lz4_flex",
+ "predicates",
  "rand",
  "regex",
  "reqwest",
diff --git a/Cargo.toml b/Cargo.toml
index 6108bb8..c6c5eab 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -35,6 +35,7 @@ uuid = { version = "1.11.0", features = ["v4"] }
 assert_cmd = "2.0.16"
 hex-literal = "0.4.1"
 insta = "1.41.1"
+predicates = "3.1.2"
 similar-asserts = "1.6.0"
 tempfile = "3"
 
diff --git a/src/mails.rs b/src/mails.rs
index 0ffcbf0..2d9ea9f 100644
--- a/src/mails.rs
+++ b/src/mails.rs
@@ -57,6 +57,7 @@ impl Mail {
         client: &Client,
         session: &Session,
         folder: &Folder,
+        ignore_new_mails: bool,
     ) -> impl Stream<Item = Result<Arc<Self>>> {
         let group_keys = Arc::clone(&session.group_keys);
         let folder_id = folder.id.clone();
@@ -69,18 +70,36 @@ impl Mail {
                 let group_keys = Arc::clone(&group_keys);
                 let folder_id = folder_id.clone();
                 async move {
-                    let mail = Self::decode(m, &group_keys, folder_id)?;
-                    Ok(Arc::new(mail))
+                    Self::decode(m, &group_keys, folder_id)
+                }
+            })
+            .try_filter_map(move |mail| async move {
+                match mail {
+                    Some(mail) => Ok(Some(Arc::new(mail))),
+                    None if ignore_new_mails => Ok(None),
+                    None => bail!("Folder contains new mail that has not been decoded before. Use the official app and view the folder to decode the data, or pass --ignore-new-mails to skip new emails."),
                 }
             })
     }
 
-    fn decode(resp: MailReponse, group_keys: &GroupKeys, folder_id: String) -> Result<Self> {
+    /// Decode [`MailReponse`].
+    ///
+    /// Returns [`None`] if no encryption key is set. This usually happens when the mail was NOT
+    /// processed via the official app yet.
+    fn decode(
+        resp: MailReponse,
+        group_keys: &GroupKeys,
+        folder_id: String,
+    ) -> Result<Option<Self>> {
+        let Some(key) = resp.owner_enc_session_key else {
+            return Ok(None);
+        };
+
         let session_key = decrypt_key(
             group_keys
                 .get(&resp.owner_group)
                 .context("getting owner group key")?,
-            resp.owner_enc_session_key,
+            key,
         )
         .context("decrypting session key")?;
 
@@ -100,7 +119,7 @@ impl Mail {
             }
         };
 
-        Ok(Self {
+        Ok(Some(Self {
             folder_id,
             mail_id: resp.id[1].clone(),
             archive_id,
@@ -111,7 +130,7 @@ impl Mail {
             subject,
             sender,
             attachments: resp.attachments,
-        })
+        }))
     }
 
     pub(crate) fn ui_url(&self) -> String {
diff --git a/src/main.rs b/src/main.rs
index d4510b1..60bfc92 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -20,6 +20,8 @@ use tracing::{debug, info};
 #[cfg(test)]
 use assert_cmd as _;
 #[cfg(test)]
+use predicates as _;
+#[cfg(test)]
 use similar_asserts as _;
 #[cfg(test)]
 use tempfile as _;
@@ -79,6 +81,13 @@ struct DownloadCLIConfig {
     /// Target path.
     #[clap(long, action)]
     path: PathBuf,
+
+    /// Ignore new mails that cannot be decrypted (yet).
+    ///
+    /// Use the official app to view and respective folder. This will convert the mail data to a
+    /// format that we can read.
+    #[clap(long, action)]
+    ignore_new_mails: bool,
 }
 
 /// Command
@@ -149,7 +158,7 @@ async fn exec_cmd(client: &Client, session: &Session, cmd: Command) -> Result<()
                 .context("folder not found")?;
             debug!(mails = folder.mails.as_str(), "download mails from folder");
 
-            Mail::list(client, session, &folder)
+            Mail::list(client, session, &folder, cfg.ignore_new_mails)
                 .map(|mail| {
                     let cfg = &cfg;
 
diff --git a/src/proto/messages.rs b/src/proto/messages.rs
index 964eb48..88242a2 100644
--- a/src/proto/messages.rs
+++ b/src/proto/messages.rs
@@ -156,7 +156,7 @@ pub(crate) struct MailReponse {
     pub(crate) _format: Format<0>,
 
     #[serde(rename = "_ownerEncSessionKey")]
-    pub(crate) owner_enc_session_key: EncryptedKey,
+    pub(crate) owner_enc_session_key: Option<EncryptedKey>,
 
     #[serde(rename = "_ownerGroup")]
     pub(crate) owner_group: String,
diff --git a/tests/cli.rs b/tests/cli.rs
index fb360de..3b98a61 100644
--- a/tests/cli.rs
+++ b/tests/cli.rs
@@ -115,4 +115,34 @@ mod integration {
             similar_asserts::assert_eq!(actual_content, expected_content);
         }
     }
+
+    #[test]
+    fn test_new_mail_without_flag() {
+        let path = TempDir::new().unwrap();
+
+        let mut cmd = cmd();
+        cmd.arg("-vv")
+            .arg("download")
+            .arg("--folder=Inbox")
+            .arg("--path")
+            .arg(path.path())
+            .assert()
+            .failure()
+            .stderr(predicates::str::contains("--ignore-new-mails"));
+    }
+
+    #[test]
+    fn test_new_mail_with_flag() {
+        let path = TempDir::new().unwrap();
+
+        let mut cmd = cmd();
+        cmd.arg("-vv")
+            .arg("download")
+            .arg("--folder=Inbox")
+            .arg("--path")
+            .arg(path.path())
+            .arg("--ignore-new-mails")
+            .assert()
+            .success();
+    }
 }