Add mail server connection check

If a `JavaMailSenderImpl` is available, check that the underlying mail
server is available on startup. Add a `spring.mail.test-connection`
property to control this behaviour.

Closes gh-3408

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mail/MailProperties.java

@@ -67,6 +67,11 @@ public class MailProperties {
 	 */
 	private String jndiName;
 
+	/**
+	 * Ping to mail server while initializing.
+	 */
+	private boolean testConnection;
+
 	public String getHost() {
 		return this.host;
 	}
@@ -119,4 +124,11 @@ public class MailProperties {
 		return this.jndiName;
 	}
 
+	public boolean isTestConnection() {
+		return this.testConnection;
+	}
+
+	public void setTestConnection(boolean testConnection) {
+		this.testConnection = testConnection;
+	}
 }

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mail/MailSenderAutoConfiguration.java

@@ -20,6 +20,7 @@ import java.util.Map;
 import java.util.Properties;
 
 import javax.activation.MimeType;
+import javax.mail.MessagingException;
 import javax.mail.Session;
 import javax.mail.internet.MimeMessage;
 
@@ -69,9 +70,21 @@ public class MailSenderAutoConfiguration {
 		else {
 			applyProperties(sender);
 		}
+		validateConnection(sender);
 		return sender;
 	}
 
+	private void validateConnection(JavaMailSenderImpl sender) {
+		if (this.properties.isTestConnection()) {
+			try {
+				sender.testConnection();
+			} catch (MessagingException ex) {
+				throw new IllegalStateException(
+						String.format("Unable to ping to %s", this.properties.getHost()), ex);
+			}
+		}
+	}
+
 	private void applyProperties(JavaMailSenderImpl sender) {
 		sender.setHost(this.properties.getHost());
 		if (this.properties.getPort() != null) {

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -519,6 +519,7 @@ content into your application; rather pick only the properties that you need.
 	spring.mail.default-encoding=UTF-8 # encoding to use for MimeMessages
 	spring.mail.properties.*= # properties to set on the JavaMail session
 	spring.mail.jndi-name= # JNDI location of a Mail Session
+	spring.mail.test-connection= # Enable test connection. Default: false
 
 	# SPRING BATCH ({sc-spring-boot-autoconfigure}/batch/BatchProperties.{sc-ext}[BatchProperties])
 	spring.batch.job.names=job1,job2