Postfix: Dictionary Attacks from multiple IPs

One of our hosted domains is receiving dictionary attacks from the various IPs and mail servers worldwide, in the form of:

Name.Surname@ourdomain

Code: 
Aug 23 18:04:41 mail postfix/smtpd[24165]: connect from mx2.aiso.net[216.132.24.8]
Aug 23 18:04:42 mail gps[24170]: ok: '' -> 'Leonardo.Grimes@ourdomain', '216.132.24.8' (0, 1627 secs)
Aug 23 18:04:42 mail postfix/smtpd[24165]: NOQUEUE: reject: RCPT from mx2.aiso.net[216.132.24.8]:
550 <Leonardo.Grimes@ourdomain>: Recipient address rejected: User unknown in virtual mailbox table;
from=<> to=<Leonardo.Grimes@ourdomain> proto=ESMTP helo=<mx2.aiso.net>
Aug 23 18:04:42 mail postfix/smtpd[24165]: disconnect from mx2.aiso.net[216.132.24.8]
There are a zillion different IPs, every attempt comes from a different IP - all ISPs and webhosts.

This lasts for days, originating probably from the same person (stupid and unskilled spammer-wannabe).

Doesn't produce any loads because this script is making one attempt at a time, approximately every minute or two.

But is extremely annoying, logs are full of this.

I am not sure if this could be prevented. Any hints?

 

 

 

 

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