As part of the anti-spam package, Postfix can perform sender address verification, through declarations like " reject_unknown_sender_domain". What you are seeing in your logs is spam-activity, most likely from a compromised PC that is acting as a spam zombie, such as the conficker worm which is famous for sending out this crap.
The first example shows in your logs, a conenction from 188.8.131.52 In your logs, it shows a connection was made and then dropped possibly because your system didn't respond in the (vulnerable ?) manner it was looking for. If we do a lookup on this address, we see that it is an unnamed host in a range belonging to the Viettel Corporation in Vietnam.
In the case of the second one, we see that a connection was made from 184.108.40.206. A reverse lookup shows this to be a DSL connection from sta. Googling this name shows that it is a blacklisted domain, possibly in Mexico, but trying to perform a lookup of this domain failed, which is an indication of a possible SPAM host.