gmail automatically parse that eml message and append content with original message
It doesn't matter how Gmail displays this message.
Messages are downloaded from IMAP in raw, unprocessed format.
Problem occurs when we trying to download attachments for such message, we are not getting file extracted from eml attachments.
Downloading and parsing are 2 separate steps.
Unless you get an exception from GetMessageByUID you don't have any kind of downloading problem.
Mail.dll processed attached emls only under very specific circumstances, for example when message/rfc822 is a single root in email's MIME tree.
Generally Mail.dll doesn't process attachments (including attached eml files) in any way.
(Of course ical, vcard entries are processed and added to appropriate IMail collections)
Consider using IMail.ExtractAttachmentsFromInnerMessages if you want to get all attachments from the message as well as attachments from all attached eml files.
If you sill believe Mail.dll is not behaving properly, please follow these steps to report a parsing problem:
https://www.limilabs.com/blog/i-have-problems-parsing-the-message