You may have hit either bug 8263 (message fragment displays incorrectly) or bug 8295 (mimehandler should be explicitly set for text/plain):
http://bugzilla.zimbra.com/show_bug.cgi?id=8263 http://bugzilla.zimbra.com/show_bug.cgi?id=8295
Both these bugs are fixed in 3.1.4, which will be released next week. If you need a shorter-term workaround, you can update the text/plain MIME handler configuration in LDAP:
Code:
# text/plain, mime, config, zimbra
dn: cn=text/plain,cn=mime,cn=config,cn=zimbra
zimbraMimeType: text/plain
cn: text/plain
objectClass: zimbraMimeEntry
zimbraMimeIndexingEnabled: TRUE
zimbraMimeHandlerClass: com.zimbra.cs.mime.handler.TextPlainHandler
zimbraMimeFileExtension: text
zimbraMimeFileExtension: txt
description: Plain Text Document
and reindex the mailbox.