I wrote a simple C# prog, to reproduce the error.
Mail.dll: 10 13:01:06 3.0.15091.1036
Mail.dll: 10 13:01:06 Checking if license file is present.
Mail.dll: 10 13:01:12 Connecting to 'smtp.rackruether.de:25', SSL: False.
Mail.dll: 10 13:01:13 S: 220 mi3 ESMTP
Mail.dll: 10 13:01:13 C: EHLO [127.0.0.1]
Mail.dll: 10 13:01:13 S: 250-mailin03.ims-firmen.de Hello x4d08cbb6.dyn.telefonica.de [77.8.203.182]
Mail.dll: 10 13:01:13 S: 250-SIZE 1048576000
Mail.dll: 10 13:01:13 S: 250-8BITMIME
Mail.dll: 10 13:01:13 S: 250-PIPELINING
Mail.dll: 10 13:01:13 S: 250-AUTH PLAIN LOGIN CRAM-MD5
Mail.dll: 10 13:01:13 S: 250-STARTTLS
Mail.dll: 10 13:01:13 S: 250 HELP
Mail.dll: 10 13:01:21 C: AUTH CRAM-MD5
Mail.dll: 10 13:01:22 S: 334 [REMOVED]
Mail.dll: 10 13:01:22 C: [REMOVED]==
Mail.dll: 10 13:01:22 S: 235 Authentication succeeded
Mail.dll: 10 13:01:41 C: MAIL FROM:<rackruether.de48@rackruether.de>
Mail.dll: 10 13:01:41 C: RCPT TO:<hommel.trading@gmx.de>
Mail.dll: 10 13:01:41 C: NOOP
Mail.dll: 10 13:01:41 S: 250 OK
Mail.dll: 10 13:01:41 S: 537-ICIR37 - unsolicited address for this account
Mail.dll: 10 13:01:41 S: 537 (
http://postmaster.config-server.de/ICIR37/77.8.203.182)
Mail.dll: 10 13:01:41 S: 250 OK
Mail.dll: 10 13:01:41 C: RSET
Mail.dll: 10 13:01:41 S: 250 Reset OK