Version:           1.8 (using KDE 3.4.0 Level "b" , SUSE 9.3)
Compiler:          gcc version 3.3.5 20050117 (prerelease) (SUSE Linux)
OS:                Linux (i686) release 2.6.11.4-21.9-default

When I am sending a mail, I got the following message:

Sending failed:
Unknown error code 50
Your server does not support sending of 8-bit messages.
Please use base64 or quoted-printable encoding.
Please send a full bug report at http://bugs.kde.org.
The message will stay in the 'outbox' folder until you either fix the problem (e.g. a broken address) or remove the message from the 'outbox' folder.
The following transport protocol was used: ...

I was trying to send it via a JAMES SMTP server.

The following is the captured tethereal output:

1301.055536 10.65.80.129 -> 213.132.43.190 TCP 52594 > smtp [SYN] Seq=0 Ack=0 Win=5840 Len=0 MSS=1460 TSV=14810559 TSER=0 WS=2
1301.060352 213.132.43.190 -> 10.65.80.129 TCP smtp > 52594 [SYN, ACK] Seq=0 Ack=1 Win=5792 Len=0 MSS=1460 TSV=1599135854 TSER=14810559 WS=0
1301.060384 10.65.80.129 -> 213.132.43.190 TCP 52594 > smtp [ACK] Seq=1 Ack=1 Win=5840 Len=0 TSV=14810564 TSER=1599135854
1301.067348 213.132.43.190 -> 10.65.80.129 SMTP Response: 220 x.bilogicsys.com SMTP Server (JAMES SMTP Server 2.2.0) ready Sun, 18 Sep 2005 12:31:34 +0400 (GST)
1301.067367 10.65.80.129 -> 213.132.43.190 TCP 52594 > smtp [ACK] Seq=1 Ack=103 Win=5840 Len=0 TSV=14810571 TSER=1599135862
1301.083533 10.65.80.129 -> 213.132.43.190 SMTP Command: EHLO [10.65.80.129]
1301.085337 213.132.43.190 -> 10.65.80.129 TCP smtp > 52594 [ACK] Seq=103 Ack=22 Win=5792 Len=0 TSV=1599135879 TSER=14810587
1301.085711 213.132.43.190 -> 10.65.80.129 SMTP Response: 250-x.bilogicsys.com Hello [10.65.80.129] (80.227.58.2 [80.227.58.2])
1301.092941 10.65.80.129 -> 213.132.43.190 SMTP Command: AUTH PLAIN ...
1301.096706 213.132.43.190 -> 10.65.80.129 SMTP Response: 235 Authentication Successful
1301.136590 10.65.80.129 -> 213.132.43.190 TCP 52594 > smtp [ACK] Seq=79 Ack=247 Win=5840 Len=0 TSV=14810641 TSER=1599135890
1301.325322 10.65.80.129 -> 213.132.43.190 SMTP Command: QUIT
1301.326688 213.132.43.190 -> 10.65.80.129 SMTP Response: 221 x.bilogicsys.com Service closing transmission channel
1301.326710 10.65.80.129 -> 213.132.43.190 TCP 52594 > smtp [ACK] Seq=85 Ack=304 Win=5840 Len=0 TSV=14810831 TSER=1599136121
1301.326715 213.132.43.190 -> 10.65.80.129 TCP smtp > 52594 [FIN, ACK] Seq=304 Ack=85 Win=5792 Len=0 TSV=1599136121 TSER=14810829
1301.327213 10.65.80.129 -> 213.132.43.190 TCP 52594 > smtp [FIN, ACK] Seq=85 Ack=305 Win=5840 Len=0 TSV=14810831 TSER=1599136121
1301.328686 213.132.43.190 -> 10.65.80.129 TCP smtp > 52594 [ACK] Seq=305 Ack=86 Win=5792 Len=0 TSV=1599136123 TSER=14810831

I can't see anything unnatural in the SMTP server's replies. (I also tried after disabling the SMTP AUTH on the server but still get the same error message from KMail)