Mails considered "ill-formed" are mails whose construction does not respect the RFC form (which applies to the construction of a mail). This is quite common when mail headers contain accented characters, for example. However, this type of mail is not as dangerous as it used to be. You can therefore decide to have this type of mail delivered. To do this, go to the "Configuration" part of the administration console, then choose "deliver" in the "Non-conforming" section.
Messages that fall into this
category do not comply with the SMTP standards described in RFC 821.
This type of message is difficult to
analyse and can therefore represent a threat.