Thansk for your answer.
Here a log from the mail with the error:
2014-11-06 13:06:17.191036500 25100 250-PIPELINING
2014-11-06 13:06:17.191036500 25100 250-8BITMIME
2014-11-06 13:06:17.191037500 25100 250-SIZE 35000000
2014-11-06 13:06:17.191083500 25100 250 AUTH PLAIN LOGIN
2014-11-06 13:06:17.259971500 25100 dispatching AUTH PLAIN AGJ1ZXR0bmVyAFBlbWlydTk0IQ==
2014-11-06 13:06:17.259973500 25100 auth::auth_cvm_unix_local plugin (auth-plain): authcvm/plain authentication attempt for: user100
2014-11-06 13:06:17.261638500 25100 235 Authentication successful for user100 - authcvm/plain
2014-11-06 13:06:17.261840500 25100 Authentication successful for user100 - authcvm/plain
2014-11-06 13:06:17.263169500 25100 tls plugin (init): ciphers: HIGH:!SSLv2:!ADH:!aNULL:!MD5:!RC4
2014-11-06 13:06:17.334037500 25100 dispatching MAIL FROM:<ulrich.user100@domain.de> SIZE=5013632
2014-11-06 13:06:17.334039500 25100 full from_parameter: FROM:<ulrich.user100@domain.de> SIZE=5013632
2014-11-06 13:06:17.334040500 25100 getting mail from <ulrich.user100@domain.de>
2014-11-06 13:06:17.334041500 25100 250 <ulrich.user100@domain.de>, sender OK - how exciting to get mail from you!
2014-11-06 13:06:17.401820500 25100 dispatching RCPT TO:<user@url.com>
2014-11-06 13:06:17.401822500 25100 250 <user@url.com>, recipient ok
2014-11-06 13:06:17.466638500 25100 dispatching DATA
2014-11-06 13:06:17.466639500 25100 354 go ahead
2014-11-06 13:06:17.690030500 25100 spooling message to disk
2014-11-06 13:07:16.621219500 25100 virus::clamav plugin (data_post): Changing permissions on file to permit scanner access
2014-11-06 13:07:16.669036500 25100 virus::clamav plugin (data_post): clamscan results: /var/spool/qpsmtpd/1415275577:25100:0: Can't allocate memory ERROR
2014-11-06 13:07:16.669038500 25100 virus::clamav plugin (data_post): ClamAV error: /usr/bin/clamdscan --stdout --config-file=/etc/clamd.conf --no-summary /var/spool/qpsmtpd/1415275577:25100:0 2>&1: 2
2014-11-06 13:07:16.669040500
2014-11-06 13:07:16.669040500 25100 logging::logterse plugin (deny): ` 88.71.1.77 dslb-088-071-001-077.088.071.pools.vodafone-ip.de [53.104.24.143] <ulrich.user100@domain.de> <user@url.com> virus::clamav 902 msg denied before queued
2014-11-06 13:07:16.669102500 25100 452 Message denied temporarily
2014-11-06 13:08:54.269567500 25100 dispatching QUIT
2014-11-06 13:08:54.270543500 25100 221 brammer-automobile.de closing connection. Have a wonderful day.
And here are the results:
[root@bmmail ~]# config show qmail
qmail=service
MaxMessageSize=35000000
status=enabled
[root@bmmail ~]# config show qpsmtpd
qpsmtpd=service
Bcc=disabled
BccMode=cc
BccUser=maillog
DNSBL=enabled
LogLevel=6
MaxScannerSize=35000000
RBLList=zen.spamhaus.org
RHSBL=enabled
RelayRequiresAuth=enabled
SBLList=multi.surbl.org:black.uribl.com:rhsbl.sorbs.net
TlsBeforeAuth=1
access=public
qplogsumm=disabled
status=enabled
[root@bmmail ~]# config show clamav
clamav=service
ArchiveBlockEncrypted=no
Checks=24
DNSDatabaseInfo=current.cvd.clamav.net
DatabaseMirror=db.local.clamav.net
Debug=no
DetectBrokenExecutables=no
FilesystemScan=daily
FilesystemScanExclude=/proc,/sys,/usr/share,/var
FilesystemScanFilesystems=/home/e-smith/files
FilesystemScanReportTo=admin
Foreground=yes
HTTPProxyPassword=
HTTPProxyPort=
HTTPProxyServer=
HTTPProxyUsername=
HeuristicScanPrecedence=no
IdleTimeout=60
LeaveTemporaryFiles=no
LogClean=no
LogFileUnlock=yes
LogTime=no
LogVerbose=yes
MaxAttempts=6
MaxConnectionQueueLength=30
MaxDirectoryRecursion=20
MaxFileSize=35M
MaxFiles=1500
MaxRecursion=8
MaxThreads=20
Quarantine=enabled
QuarantineDirectory=/var/spool/clamav/quarantine
ReadTimeout=300
ScanArchive=yes
ScanHTML=yes
ScanMail=yes
ScanOLE2=yes
ScanPE=yes
ScanRAR=no
SelfCheck=1800
ShowProxySettings=no
ShowUpdateSettings=no
SignaturesUpdated=unknown
UpdateNonOfficeHrs=disabled
UpdateOfficeHrs=disabled
UpdateWeekend=disabled
status=enabled
[root@bmmail ~]# config show php
php=service
AllowUrlFopen=Off
UploadMaxFilesize=35M
status=enabled
[root@bmmail ~]# config show spamassassin
spamassassin=service
BayesAutoLearnThresholdNonspam=0.10
BayesAutoLearnThresholdSpam=4.00
DNSAvailable=yes
MaxMessageSize=35000000
MessageRetentionTime=90
OkLanguages=all
OkLocales=all
RejectLevel=15
ReportSafe=0
Sensitivity=custom
SkipRBLChecks=0
SortSpam=enabled
Subject=[SPAM]
SubjectTag=enabled
TagLevel=10
UseBayes=1
status=enabled