Pegar propriedades de um arquivo externo com o Spring

mail.properties

mail.smtp.host=smtp.gmail.com
mail.smtp.port=587
mail.smtp.username=seu-email-aqui@gmail.com
mail.smtp.password=suasenhaaqui

MailConfig.java

package com.algaworks.email.config;

import java.util.Properties;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.PropertySource;
import org.springframework.core.env.Environment;
import org.springframework.mail.javamail.JavaMailSender;
import org.springframework.mail.javamail.JavaMailSenderImpl;

@Configuration
@PropertySource("file:/home/usuario/mail.properties") // ou, mais prático usar: "file:${user.home}/mail.properties"
public class MailConfig {
	
	@Autowired
	private Environment env;
	
	@Bean
	public JavaMailSender mailSender() {
		JavaMailSenderImpl mailSender = new JavaMailSenderImpl();
		
		mailSender.setHost(env.getProperty("mail.smtp.host"));
		mailSender.setPort(env.getProperty("mail.smtp.port", Integer.class));
		mailSender.setUsername(env.getProperty("mail.smtp.username"));
		mailSender.setPassword(env.getProperty("mail.smtp.password"));
		
		Properties props = new Properties();
		props.put("mail.transport.protocol", "smtp");
		props.put("mail.smtp.auth", true);
		props.put("mail.smtp.starttls.enable", true);
		props.put("mail.smtp.connectiontimeout", 10000);
		
		mailSender.setJavaMailProperties(props);
		
		return mailSender;
	}
}