When an mbox email contains a line that looks like a "Mail" header it should be quoted (per RFC 4155). Sendmail's delivery agent does this, as does Procmail delivery agent. Dovecot deliver does not do this.
Example line, with ">" quote:
From hokan@hokan.org Thu Mar 8 17:28:01 2008
When reading email (IMAP) that contains such an unquoted line, the email abruptly ends before the "Mail" line. There may (or may not) appear another email (visible to the client) that contains the rest of the email.
-- Hokan MEnet, a wholly owned subsidiary of Enet System Administrator Department of Mechanical Engineering hokan@me.umn.edu Institute of Technology 612.626.9800 University of Minnesota