When you use Microsoft BizTalk Server to process email messages from Microsoft Exchange Server, you may encounter an error stating that a message is invalid. This issue may occur if the message has a header field that's longer than 1,000 characters.
Cumulative update information
The fix that resolves this issue is included in Cumulative Update 3 for BizTalk Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about service pack and cumulative update list for BizTalk Server.
Learn about BizTalk Server hotfixes and cumulative update support.