[Bug 3192] New: Long message headers are corrupted when bouncing

https://bugs.exim.org/show_bug.cgi?id=3192

            Bug ID: 3192
           Summary: Long message headers are corrupted when bouncing
           Product: Exim
           Version: 4.99
          Hardware: x86
                OS: Linux
            Status: NEW
          Severity: bug
          Priority: medium
         Component: Delivery in general
          Assignee: [email protected]
          Reporter: [email protected]
                CC: [email protected]

Created attachment 1531
  --> https://bugs.exim.org/attachment.cgi?id=1531&action=edit
Original message

When bouncing a message with long (998+ characters long) headers, even if
appropriately wrapped according to MIME RFCs, not only the original header is
truncated, but the next header is effectively deleted. This repeats if the
bounce is resent.

For example, when this message is bounced: 

======================
Subject: test
X-Long-Header:  A01 B01 C01 D01 E01 F01 G01 H01 I01 J01 K01 L01 M01 N01
 A02 B02 C02 D02 E02 F02 G02 H02 I02 J02 K02 L02 M02 N02
 A03 B03 C03 D03 E03 F03 G03 H03 I03 J03 K03 L03 M03 N03
 ....
 A23 B23 C23 D23 E23 F23 G23 H23 I23 J23 K23 L23 M23 N23
Content-Type: text-plain
MIME-Version: 1.0
Date: Wed, 21 Jan 2026 17:22:32 +0000

Hello 
======================

In the first bounce, Content-Type suffers:
======================
Subject: test
X-Long-Header: A01 B01 C01 D01 E01 ...
 N07 A08 B08 C08 D08 E08 F08... 
 C15 D15 E15... E18 F18 G18Content-Type: text-plain
MIME-Version: 1.0
Date: Wed, 21 Jan 2026 17:22:32 +0000

Hello
======================

If this is resent, as the resulting header is longer than 998 characters, the
next header is affected:

======================
Subject: test
X-Long-Header: A01 B01 C01 D01 E01 ...
 N07 A08 B08 C08 D08 E08 F08 ...
 C15 D15 E15 .... C18 D18 E18 F18MIME-Version: 1.0
Date: Wed, 21 Jan 2026 17:22:32 +0000

Hello
======================

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