[blinki bill]

you can add some more security like rejecting submissions with "illegal" characters and/or html tags like "<a>" inside.
Other thing is to improve it against bots - one of the best solutions i saw till now is trough js where it checks the visitor screen width (bots emulate pretty everything to look like real people but this value is always "0") and if it's not a human the submit button just doesn't comes on the page (way more successful method than the catchpa)

One other thing in your case the domains submitted doesn't exists so if you have a recip link checker and the page submitted doesn't have your recip or doesn't exists it won't be added to the base