GH-3560 Parse mail FROM as comma-delimited header (#3562)

Fixes https://github.com/spring-projects/spring-integration/issues/3560

According RFC 5322 `FROM` and `REPLY-TO` received mail message can be
as an array of addresses.

* Fix `MailUtils` to present those arrays as comma-delimited strings for
Spring message headers
* Fix tests to deal already with several addresses for `FROM` mime header.

In the future we may change the logic to map those mime headers to arrays
as it states according the mentioned RFC

**Cherry-pick to `5.4.x` & `5.3.x`**

spring-integration-mail/src/main/java/org/springframework/integration/mail/support/MailUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2019 the original author or authors.
+ * Copyright 2016-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,12 +25,14 @@ import javax.mail.Message.RecipientType;
 
 import org.springframework.integration.mail.MailHeaders;
 import org.springframework.messaging.MessagingException;
-import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
  * Utilities for handling mail messages.
  *
  * @author Gary Russell
+ * @author Artem Bilan
+ *
  * @since 4.3
  *
  */
@@ -47,29 +49,21 @@ public final class MailUtils {
 	 * @return the map.
 	 */
 	public static Map<String, Object> extractStandardHeaders(Message source) {
-		Map<String, Object> headers = new HashMap<String, Object>();
+		Map<String, Object> headers = new HashMap<>();
 		try {
-			headers.put(MailHeaders.FROM, convertToString(source.getFrom()));
+			headers.put(MailHeaders.FROM, StringUtils.arrayToCommaDelimitedString(source.getFrom()));
 			headers.put(MailHeaders.BCC, convertToStringArray(source.getRecipients(RecipientType.BCC)));
 			headers.put(MailHeaders.CC, convertToStringArray(source.getRecipients(RecipientType.CC)));
 			headers.put(MailHeaders.TO, convertToStringArray(source.getRecipients(RecipientType.TO)));
-			headers.put(MailHeaders.REPLY_TO, convertToString(source.getReplyTo()));
+			headers.put(MailHeaders.REPLY_TO, StringUtils.arrayToCommaDelimitedString(source.getReplyTo()));
 			headers.put(MailHeaders.SUBJECT, source.getSubject());
 			return headers;
 		}
-		catch (Exception e) {
-			throw new MessagingException("conversion of MailMessage headers failed", e);
+		catch (Exception ex) {
+			throw new MessagingException("conversion of MailMessage headers failed", ex);
 		}
 	}
 
-	private static String convertToString(Address[] addresses) {
-		if (addresses == null || addresses.length == 0) {
-			return null;
-		}
-		Assert.state(addresses.length == 1, "expected a single value but received an Array");
-		return addresses[0].toString();
-	}
-
 	private static String[] convertToStringArray(Address[] addresses) {
 		if (addresses != null) {
 			String[] addressStrings = new String[addresses.length];

spring-integration-mail/src/test/java/org/springframework/integration/mail/ImapMailReceiverTests.java

@@ -709,7 +709,7 @@ public class ImapMailReceiverTests {
 
 	@Test // see INT-1801
 	public void testImapLifecycleForRaceCondition() throws Exception {
-		for (int i = 0; i < 1000; i++) {
+		for (int i = 0; i < 100; i++) {
 			final ImapMailReceiver receiver = new ImapMailReceiver("imap://foo");
 			Store store = mock(Store.class);
 			Folder folder = mock(Folder.class);

spring-integration-mail/src/test/java/org/springframework/integration/mail/config/Pop3Tests.java

@@ -67,7 +67,7 @@ public class Pop3Tests {
 		assertThat(headers.get(MailHeaders.TO, String[].class)[0]).isEqualTo("Foo <foo@bar>");
 		assertThat(Arrays.toString(headers.get(MailHeaders.CC, String[].class))).isEqualTo("[a@b, c@d]");
 		assertThat(Arrays.toString(headers.get(MailHeaders.BCC, String[].class))).isEqualTo("[e@f, g@h]");
-		assertThat(headers.get(MailHeaders.FROM)).isEqualTo("Bar <bar@baz>");
+		assertThat(headers.get(MailHeaders.FROM)).isEqualTo("Bar <bar@baz>,Bar2 <bar2@baz>");
 		assertThat(headers.get(MailHeaders.SUBJECT)).isEqualTo("Test Email");
 		assertThat(message.getPayload()).isEqualTo("foo\r\n\r\n");
 	}

spring-integration-mail/src/test/java/org/springframework/integration/mail/dsl/MailTests.java

@@ -142,7 +142,7 @@ public class MailTests {
 		assertThat(message).isNotNull();
 		MessageHeaders headers = message.getHeaders();
 		assertThat(headers.get(MailHeaders.TO, String[].class)).containsExactly("Foo <foo@bar>");
-		assertThat(headers.get(MailHeaders.FROM)).isEqualTo("Bar <bar@baz>");
+		assertThat(headers.get(MailHeaders.FROM)).isEqualTo("Bar <bar@baz>,Bar2 <bar2@baz>");
 		assertThat(headers.get(MailHeaders.SUBJECT)).isEqualTo("Test Email");
 		assertThat(message.getPayload()).isEqualTo("foo\r\n\r\n");
 		assertThat(message.getHeaders().containsKey(IntegrationMessageHeaderAccessor.CLOSEABLE_RESOURCE)).isTrue();

spring-integration-test-support/src/main/java/org/springframework/integration/test/mail/TestMailServer.java

@@ -484,11 +484,11 @@ public final class TestMailServer {
 
 			public static final String MESSAGE =
 					"To: Foo <foo@bar>\r\n"
-					+ "cc: a@b, c@d\r\n"
-					+ "bcc: e@f, g@h\r\n"
-					+ "From: Bar <bar@baz>\r\n"
-					+ "Subject: Test Email\r\n"
-					+ "\r\n" + BODY;
+							+ "cc: a@b, c@d\r\n"
+							+ "bcc: e@f, g@h\r\n"
+							+ "From: Bar <bar@baz>, Bar2 <bar2@baz>\r\n"
+							+ "Subject: Test Email\r\n"
+							+ "\r\n" + BODY;
 
 			protected final Socket socket; // NOSONAR protected