Assuming you're using Spring 2.5 or better, this is how the configuration should read:
Code:
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-2.5.xsd">
<beans>
<context:property-placeholder location="classpath:smtp_config.properties"/>
<bean id="mailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl">
<property name="host" value="${smtp.host}"/>
</bean>
</beans>
If this does not work for you, please attach sample code, preferably buildable with Maven.