Add the property codes below in designer.tra (under <TIBCO-HOME>\designer\5.6\bin), restart the designer and retest.
Trace.Task.*=true
Trace.Debug.*=true
java.property.mail.debug=true
java.extended.properties=-Dmail.mime.base64.ignoreerrors=true
bw.plugin.mail.receiverHandleDiscreteTypes=true
bw.engine.showInput=true
bw.engine.showOutput=true
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