Horde IMP 4.3.6 and earlier does not request that the web browser avoid DNS prefetching of domain names contained in e-mail messages, which makes it easier for remote attackers to determine the network location of the webmail user by logging DNS requests.
https://secure.grepular.com/DNS_Prefetch_Exposure_on_Thunderbird_and_Webmail
https://exchange.xforce.ibmcloud.com/vulnerabilities/56052