In most of the cases “This is a multipart message in MIME format” means the email message includes 2 content versions: HTML and text-only.

I tested your DXL with PD4ML v381 – the build suppresses “This is a multipart message in MIME format”, however still outputs both content versions as PDF.

OutOfMemory error usually can be handled with an overriding of Max Heap Size JVM parameter (i.e. -Xmx512m). But first I would recommend to give a try to PD4ML v381 – since v380fx5 you currently use – we addressed a number of DXL converting issues.