diff --git a/README.md b/README.md
index a8dfe15..0c893c5 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,14 @@
 # gpgmymail
 
-Takes an email from stdin and encrypts it using the recipient's PGP key,
-provided as an argument when calling the script.
+Takes an email from stdin and encrypts it to stdout using the recipient's PGP
+key, provided as an argument when calling the script.
+
+Leaves a `X-gpgmymail-Status` header on the email, which has the following
+statuses:
+
+* `entered` - the email has entered the encryption function, but not been
+              encrypted
+* `encrypted` - the encryption function has encrypted the email
 
 Written to be a Sieve filter to be used with `sieve_extprograms`. Can be used
 in a Sieve filter e.g.:
diff --git a/gpgmymail b/gpgmymail
index c0acc7b..8622548 100755
--- a/gpgmymail
+++ b/gpgmymail
@@ -24,6 +24,12 @@ works well for emails created with this tool. When encrypting, the tool
 preserves all headers in the original email in the encrypted part, and
 copies relevant headers to the output. When decrypting, any headers are
 ignored, and only the encrypted headers are restored.
+
+Emails exiting this script will have the 'X-gpgmymail-Status' header, which has
+the following options:
+
+- entered: the email has entered the encrypt() function
+- encrypted: the email has been encrypted
 """
 
 import argparse
@@ -219,6 +225,9 @@ def encrypt(
     blocks. find ignore_errors to see instances where this occurs
 
     :return: The encrypted email as a string"""
+
+    # mark the email as having passed through us
+    set_email_header(message, 'X-gpgmymail-Status', 'entered')
     
     # exclusion criteria:
     # some mail clients like Thunderbird don't like twice-encrypted emails,
@@ -280,6 +289,9 @@ def encrypt(
     set_email_header(encmsg, 'Subject', PEP_SUBJECT)
     set_email_header(encmsg, 'X-pEp-Version', '2.1')
 
+    # we have encrypted the email, set our gpgmymail header appropriately
+    set_email_header(encmsg, 'X-gpgmymail-Status', 'encrypted')
+
     return encmsg.as_string()
 
 def decrypt(message: email.message.Message, *, encoding: str = DEFAULT_ENCODING) -> str: