I've recently come across some problem with Poco sending mail by SMTP that has problem beeing read by two older mail components due to the ordering of the Message headers.
The actual compnents is SEE32 from Marshalsoft and Postie from Infradig. Reading the mails with Outlook Express works.
It seems that these are depening on the Message headers coming in some special order and not in alphabetic order as POCO sends them.
I've now solved this by modifying MessageHeader::write in Net/src/MessageHeader.cpp to send headers in an order accepted by those components.
I just want to make sure that there's nothing in the SMTP / MIME / whatever standard that POCO is violating, and it's the other components failing to interpret the headers correctly.
Another reason to post this is of cause to help other POCO users to find a solution to the problem I've took me for at least a week to find.