This message comes from mail(1) or mailx(1) whenever it detects messages with a different content length than advertised. The mail(1) program tells you which message might be truncated or might have another message concatenated to it.
Two common causes of content length mismatches are the simultaneous use of different mail readers (such as mail(1) and mailtool(1)), or the use of a mail reading program (or an editor) that does not update the content-length field after altering a message.
The mailx(1) program can usually recover from this error and delineate mail message boundaries correctly. Pay close attention to the message that might be truncated or combined with another message, and to all messages after that one. If a mail file becomes hopelessly corrupted, run it through a text editor to eliminate all Content-Length lines, and ensure that each message has a From (no colon) line for each message, preceded by a blank line.