How do I check non-existing mail-adresses for reason of filtering spam guestbook entries

I created a guestbook in php, html, sql.
Entry fields are name, e-mail-adress, personal website, message.
Everything was fine with it for a good year now, but yesterday I sent an e-mail
to a friend who is located at a hotmail.com mailserver and after that different kind of people or
one or any program seems to spam my guestbook. I find it hard to filter out the
messages since the only thing 'wrong' with them is that the e-mail adresses
given do not exist they look like
 
yxyxyxgarbage@hotmail.com

and if I reply to them the mail is rejected by server because of unknown adress:

    The mail system

    <hsjgpwe@hotmail.com>: host mx4.hotmail.com[65.54.245.104] said: 550 Requested
     action not taken: mailbox unavailable (in reply to RCPT TO command)

each mail-adress is different, same as each web-adress, name  and message.
Because name, site and message given sound real and do not contain offending keywords, which I could use for filtering out,
it let me come to conclusion that messages are not really automatted and b) the
only point of distinction against 'serious' entries is the nonexisting mailbox of the visitor.
(I cannot even make a rule for the garbage part of the mail-adress, since in that part of
e-mail anything is allowed and would not make a differrence to 'existing' ones)

So, only way: is there a function I could implement, that, after pressing the 'submit' button
would check, whether mailbox does exist or not and then if not would work as a filter? Thanks.Start Free Trial