Someone sent you mail with Katna and chose to see whether it was opened and whether its links were followed. Your mail program fetched a small picture or followed a link through this server, and the sender was told when.
This server stores only a random number for each copy of the mail, the addresses of its links, and the time of each open or click, labelled as a person, Apple's privacy proxy or a security scanner. It never receives the subject, the recipients or the content of the mail, and it does not keep IP addresses or browser details. Records are deleted after a few months.
To stop it, turn off remote pictures in your mail program.