Recently I used the mime encoder in a custom pipeline and got an error when sending a message through the pipeline at runtime:
There was a failure executing the send pipeline: ........................
Reason: Retrieving the COM class factory for component with CLSID {254B4003-2AA7-4C82-BB2E-18BA7F22DCD2} failed due to the following error: 80040154
After some time I figured out that this was because the send port run under a 64bit Host Instance. I changed it to 32bit and it worked. Obviously MimeEncoder does not support 64bit - this is also stated in BizTalk Documentation:
http://msdn.microsoft.com/en-us/library/aa578427(BTS.10).aspx
http://technet.microsoft.com/en-us/library/aa560166(BTS.10).aspx