We changed a lot of mail contact Exchange accounts to Mail User accounts since they had AD accounts, but no mailboxes with us.
This has caused a lot of NDR messages due to people trying to email their old account, which is the same address, but a Different X400 address. Instead of trying to clear these cached addresses for countless uers, I've figured out take the ICMEAX out of the NDR, convert it to X500 and attach it as a proxy address. By following instructions found here http://msexchangeguru.com/2012/03/15/x500/
and other places.
However, I have one user that has a +3F in the middle of her name, and I can't seem to find what value it is supposed to be converted to.