Possibly a few spam bots there - most of the hostnames showing are styled as dynamic addresses - issued to clients by ISPs from their pool.
They probably shouldn't be attempting to connect directly to your server.
Part of anti-spam checks postfix is trying to lookup the hosts that are attempting to connect. This look is failing and thats what postfix is giving you a warning for.
pm2-24.skyweb.net - no forward DNS record exists for that host, although a reverse PTR does seem to exist for 188.8.131.52.
Here's a transcript off one of mine:
May 27 13:43:35 server1 postfix/smtpd: warning: 184.108.40.206: hostname bl22-18-167.dsl.telepac.pt verification failed: Name or service not known
May 27 13:43:35 server1 postfix/smtpd: connect from unknown[220.127.116.11]
May 27 13:43:36 server1 postfix/smtpd: NOQUEUE: reject: RCPT from unknown[18.104.22.168]: 504 5.5.2 <22.214.171.124>: Helo command rejected: need fully-qualified hostname; from=<> to=<my.email.addy> proto=SMTP helo=<126.96.36.199>
May 27 13:43:36 server1 postfix/smtpd: lost connection after RCPT from unknown[188.8.131.52]
Postfix tries its reverse lookup and fails. Client tries to connect anyway and whatever spam its trying to push is rejected by the box as reject_non_fqdn_hostname is set.
The warning on its own isn't an issue - have a read of any further postfix hardening to prevent spam that you might not already have in place.