diff --git a/lib/g3-icap-client/src/reqmod/imap/mod.rs b/lib/g3-icap-client/src/reqmod/imap/mod.rs
index 59037edf8..dcd2b3a79 100644
--- a/lib/g3-icap-client/src/reqmod/imap/mod.rs
+++ b/lib/g3-icap-client/src/reqmod/imap/mod.rs
@@ -87,10 +87,6 @@ impl<I: IdleCheck> ImapMessageAdapter<I> {
         header.extend_from_slice(len_s.as_bytes());
         header.extend_from_slice(b"\r\n");
 
-        header.extend_from_slice(b"X-IMAP-Message-Size: ");
-        header.extend_from_slice(len_s.as_bytes());
-        header.extend_from_slice(b"\r\n");
-
         header.extend_from_slice(b"\r\n");
         header
     }
diff --git a/sphinx/g3proxy/protocol/helper/icap_imap.rst b/sphinx/g3proxy/protocol/helper/icap_imap.rst
index 1af503e1f..5b3eecc74 100644
--- a/sphinx/g3proxy/protocol/helper/icap_imap.rst
+++ b/sphinx/g3proxy/protocol/helper/icap_imap.rst
@@ -29,10 +29,6 @@ The following headers will be set in the HTTP PUT request:
   The ICAP server can modify the mail message but should not change the mail message size.
   It is recommended for the ICAP server to set Content-Length as part of the HTTP request header in it's ICAP response.
 
-- X-IMAP-Message-Size
-
-  The value will be the exact size of the mail message in the IMAP APPEND command.
-
 The body of the HTTP PUT request will be the corresponding mail message data.
 
 Limitations