Mule not always sending data on outbound HTTP endpoint? - http

Question

I'm fairly new at Mule and trying to get a simple flow to work. The basic idea is to fetch email from an imap box, get the body (text) of the email, put it in a xml / soap message and post to a soap webservice.
I've got my flow set up and it seems to work for about 50% of the mails. In case the message is short, everything works as expected; in case the mail is somewhat longer or containing an attachment, it's working partially.
Here's my flow (only the relevant parts):
<imap:inbound-endpoint host="mailserver" port="143"
user="me" password="mypassword" responseTimeout="10000" doc:name="IMAP"
connector-ref="imap">
<remove-attachment attachmentName="*" />
</imap:inbound-endpoint>
<remove-attachment attachmentName="*" doc:name="Attachment" />
<logger level="INFO" doc:name="Logger" />
<set-variable variableName="mailBody" value="#[payload]"
doc:name="Variable" />
<mulexml:object-to-xml-transformer
doc:name="Object to XML" />
<mulexml:xslt-transformer maxIdleTransformers="2"
maxActiveTransformers="5" xsl-file="submit_to_wfp.xsl" doc:name="XSLT"
ignoreBadInput="true">
<mulexml:context-property key="mailbody"
value="#[mailBody]" />
</mulexml:xslt-transformer>
<object-to-string-transformer doc:name="Object to String"
encoding="UTF-8" ignoreBadInput="true" mimeType="text/xml" />
<message-properties-transformer
doc:name="Message Properties" overwrite="true" encoding="UTF-8"
mimeType="text/xml">
<add-message-property key="content-Type" value="text/xml" />
</message-properties-transformer>
<!-- <file:outbound-endpoint path="/Users/me" outputPattern="xml.txt" responseTimeout="10000"
doc:name="File"/> -->
<http:outbound-endpoint exchange-pattern="request-response"
host="localhost" port="8082" doc:name="HTTP" contentType="text/xml"
path="wfp/services/wfp" encoding="UTF-8" mimeType="text/xml"
responseTimeout="500000" />
The strange thing is that in case I enabled the outbound file endpoint in stead of HTTP, everything works and the flow yields a flawless XML file. In case of the longer message (due to lots of text and / or attachments), Mule fails by not sending any data over the HTTP endpoint (log tells me it's triggered though).
Where should I be looking? Using Mule 3.4CE btw.
Thanks!