Dynamic caching of InfoPath eMail forms must be disabled.

InfoPath caches form templates when they are attached to a mail item recognized as an InfoPath e-mail form. When users fill out forms running with a restricted security level, InfoPath uses the cached version of the mailed template, rather than any published version. To circumvent users filling out a published form, an attacker could e-mail an alternate version of the form, which would return the data to the sender as part of a phishing attack and could be used to gain access to confidential information.