While executing smtp check Zabbix server will terminate connection by sending "QUIT/n".
However, according to RFC2821 all lines should be terminated using "\r\n"( http://www.ietf.org/rfc/rfc2821.txt )
This behavior can lead to ( and in my case did ) situation when connection is not closed and next connection to SMTP server will be merged into not-closed one and SMTP server will reply with:
500 5.5.2 Error: bad syntax; smtp_resp: QUIT\nEHLO localhost.localdomain\r\n
The problem was observed with amavisd-2.7.2
Simple patch that fixes the problem with simple check for smtp attached.