diff --git a/doc/standards.md b/doc/standards.md
index fe00c99d3..f37d16ab2 100644
--- a/doc/standards.md
+++ b/doc/standards.md
@@ -109,6 +109,8 @@ The code should comply to these, but should be more compliant to existing popula
 
  - [rfc2045](https://datatracker.ietf.org/doc/html/rfc2045)
     : Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies
+ - [rfc2046](https://datatracker.ietf.org/doc/html/rfc2046)
+    : Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types
  - [rfc2047](https://datatracker.ietf.org/doc/html/rfc2047)
     : MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text
  - [rfc2231](https://datatracker.ietf.org/doc/html/rfc2231)
diff --git a/lib/g3-icap-client/src/reqmod/smtp/mod.rs b/lib/g3-icap-client/src/reqmod/smtp/mod.rs
index f9093482a..8d60d1df2 100644
--- a/lib/g3-icap-client/src/reqmod/smtp/mod.rs
+++ b/lib/g3-icap-client/src/reqmod/smtp/mod.rs
@@ -101,6 +101,7 @@ impl<I: IdleCheck> SmtpMessageAdapter<I> {
     pub fn build_http_header(&self) -> Vec<u8> {
         let mut header = Vec::with_capacity(128);
         header.extend_from_slice(b"PUT / HTTP/1.1\r\n");
+        header.extend_from_slice(b"Content-Type: message/rfc822\r\n");
         header.extend_from_slice(b"\r\n");
         header
     }