Sendmail+Cyrus IMAP "Data format error" and bounces

[Andrea Venturoli]

Hello.

I'm running sendmail 8.15.2, delivering to Cytus IMAP 3.0.13.



 From time to time, I receive some message that are refused and bounce.
Do to possible blacklisting, I *really* want to avoid bounces.



Here's a sample log:
sm-mta[97841]: 00DI1r4x097841: from=<in... at services.xxx.com>, size=37059,
class=0, nrcpts=1,
msgid=<PATT3L844_19452433_q5ZeSIsrRLtOGReCXrQXgg==.8A... at xxxxxxxxxxx.xxxxxxxx.net>,
proto=UTF8SMTPS, daemon=MTA, relay=xxxxxxxxxxx.xxxxxxxx.net [37.148.183.222]
sm-mta[97960]: AUTH=client, relay=localhost, mech=, bits=0
sm-mta[97960]: 00DI1r4x097841: to=xxxxxxxxxx, delay=00:00:11,
xdelay=00:00:00, mailer=cyrusv2, pri=67551, relay=localhost, dsn=5.6.7,
stat=Data format error
sm-mta[97960]: 00DI1r4x097841: 00DI264w097960: DSN: Data format error
sm-mta[97960]: 00DI264w097960: to=<in... at services.xxx.com>,
delay=00:00:04, xdelay=00:00:04, mailer=esmtp, pri=30000,
relay=mx1.slgnt.eu., dsn=5.1.2, stat=Host unknown (Name server:
xxx.xxxxx.xx.: no data known)



Most of these messages are spam/malware, but the above was a legitimate
message from a service provider I use.
I usually become aware of this, due to the second bounce (legitimate
mail usually comes from a "noreply" address, while spam/malware of
course from spoofed/inexistent addresses).



There's nothing more in the sendmail logs and Cyrus logs nothing about this.
I've searched the web, but found no explanation.
I've asked the sendmail side and they suggested I ask on the Cyrus-side.



Does anyone have any idea what might be the reason behind this "Data 
format error"?
Any hint on how to avoid it?


   bye & Thanks
         av.