references header corrupted

I have a long-running email exchange whose References: header is long and getting corrupted. For example:

References: <alpine.BSF.2.00.1012202115590.12464@xxxxxxxxxxxxxxxxxxx.com> <D7687367-C458-4CA8-80C0-28FBBA3A7DE6@yyyyyyyyyyy.com> <4D66D2DC.9070506@xxxxxxxxxxxxxxxxxxx> <1244C3C3-FE75-469A-8B11-E3A9A03E580B@yyyyyyyyyyy.com> <4D7A92C0.2060406@xxxxxxxxxxxxxxxxxxx.com> <006e01cbed98$95736250$c05a26f0$@com> <4D911C4F.4020001@xxxxxxxxxxxxxxxxxxx.com> <4E8F6BF4.7060409@xxxxxxxxxxxxxxxxxxx.com> <004c01cc8538$e1169c80$a343d580$@com> <4E8F762A.7040603@xxxxxxxxxxxxxxxxxxx.com> <007401cca49e$dacedd80$906c9880$@com> <4EC4313C.8030302@xxxxxxxxxxxxxxxxxxx.com> <00db01ccb917$c94aa340$5bdfe9c0$@com> <4EE6E610.4020105@xxxxxxxxxxxxxxxxxxx.com> <002c01ccb9a4$48ffa3b0$dafeeb10$@com> <4EE8C793.3050908@xxxxxxxxxxxxxxxxxxx.com> <003701ccbe76$c4444a90$4cccdfb0$@c! ! om> <4EF09EA5.8050007 @xxxxxxxxxxxxxxxxxxx.com> <001501ccbf26$f6dfdb30$e49f9190$@com> <4F28E6F! F.7030004@xxxxxxxxxxxxxxxxxxx.com>X-System-Scanned: Yes <09DDD1F9-91E2-4762-B817-3098843C2F81@yyyyyyyyyyy.com>

Note problems such as the "c! ! om" in one of the references, and the concatenation of the "X-System-Scanned" header line into the References header.

When I reply to messages like this one, the recipient sees an empty line where the References header was, which results in message filtering failures on his end because there are additional message header lines that follow.

It seems like, when sending a message in reply, Mac Mail might not be reformatting the References line into folded lines to handle the 998-character maximum header line length defined in RFC 2822. Outbound messages go through sendmail 8.14.3 and MailScanner 4.82.6; I would expect sendmail to be pretty robust, and MailScanner to be robust as well, but they could be part of the problem...