You have RosettaNet solutions deployed on BizTalk 2013 R2. After urpgrading to CU2 (or higher) or BizTalk 2016, your trading partner complained that they cannot process the documents sent from you with existing configuration due to an extra unicode Byte Order Mark(BOM) in the beginning of the doc.
We debug the issue and found the root cause is because some code change within MIMEEncoder pipeline component(in CU2). Also, setting PreserveBOM to true or false in Pipeline setting doesn't change the behavior.
We've verified that the issue only occurs when content-encoding is set to 8bit or quoted-printable in RosettaNet agreement setting. Using Base64 encoding will not hit the same problem.
The issue has been reported to BizTalk production group for further investigation and judgement about whether a fix is required.
Please understand BOM isn't unexpected character for unicode encoded content. So normally the partner should be able to configure their application to accept the documents with BOM.
If no luck, currently a work around is to switch to Base64 encoding instead of 8bit or quoted-printable(see below).