What is this kind of spam trying to do?

The contact form on my website receives some spam occasionally.

It's always a meaningless jumble of urls. It is always of this type (I've replaced the meaningless jumble with some of my own in case the urls are valid):

ty8769 <a href="http://iduqfhuerhferif.com/">iduqfhuerhferif</a>;, [url=http://powjrjie.com/]powjrjie[/url], [link=http://oproprktt.com/]oproprktt[/link], http://pesdukiqwro.com/