Massive memory usage

[voltron81]

Ok a quick update:
I've read all the files and, from syslog, I can see when the crash happens

Jul 13 17:09:32 srv3 postfix/smtpd[10562]: warning: 188.115.221.40: hostname host-188-115-221-40.orangearmenia.am verification failed: Name or service not known
Jul 13 17:09:32 srv3 postfix/smtpd[10562]: warning: 188.115.221.40: hostname host-188-115-221-40.orangearmenia.am verification failed: Name or service not known
Jul 13 17:09:32 srv3 postfix/smtpd[9564]: NOQUEUE: reject: RCPT from 189.58.69.37.dynamic.adsl.gvt.net.br[189.58.69.37]: 550 5.1.1 <part1.01070107.02010909@XXXX.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<part1.01070107.02010909@XXXX.com> to=<part1.01070107.02010909@XXXX.com> proto=SMTP helo=<189.58.69.37.dynamic.adsl.gvt.net.br>
Jul 13 17:09:32 srv3 postfix/smtpd[9564]: NOQUEUE: reject: RCPT from 189.58.69.37.dynamic.adsl.gvt.net.br[189.58.69.37]: 550 5.1.1 <part1.01070107.02010909@XXXX.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<part1.01070107.02010909@XXXX.com> to=<part1.01070107.02010909@XXXX.com> proto=SMTP helo=<189.58.69.37.dynamic.adsl.gvt.net.br>
Jul 13 17:09:32 srv3 postfix/smtpd[9564]: connect from 189.58.69.37.dynamic.adsl.gvt.net.br[189.58.69.37]
Jul 13 17:09:32 srv3 postfix/smtpd[9564]: connect from 189.58.69.37.dynamic.adsl.gvt.net.br[189.58.69.37]
Jul 13 17:09:33 srv3 postfix/smtpd[9564]: disconnect from 189.58.69.37.dynamic.adsl.gvt.net.br[189.58.69.37]
Jul 13 17:09:33 srv3 postfix/smtpd[9564]: lost connection after RCPT from 189.58.69.37.dynamic.adsl.gvt.net.br[189.58.69.37]
Jul 13 17:09:35 srv3 postfix/smtpd[10535]: disconnect from unknown[218.248.64.232]
Jul 13 17:09:50 srv3 postfix/smtpd[10669]: sql auxprop plugin using mysql engine
Jul 13 17:09:52 srv3 postfix/smtpd[10687]: sql auxprop plugin using mysql engine
Jul 13 17:09:52 srv3 postfix/smtpd[10763]: sql auxprop plugin using mysql engine
Jul 13 17:09:52 srv3 postfix/smtpd[10764]: sql auxprop plugin using mysql engine
Jul 13 17:09:52 srv3 postfix/smtpd[10765]: sql auxprop plugin using mysql engine
Jul 13 17:09:52 srv3 postfix/smtpd[10766]: sql auxprop plugin using mysql engine
Jul 13 17:09:52 srv3 postfix/smtpd[10767]: sql auxprop plugin using mysql engine

Any idea of what is going on?
I really want to understand how the server crashed, so to avoid another crash for the same reason...
thanks
Michele