Cause

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.