Explicitly mention Jakarta Mail 1.6 next to JavaMail

See gh-25855

src/docs/asciidoc/integration.adoc

@@ -4421,10 +4421,11 @@ This section describes how to send email with the Spring Framework.
 The following JAR needs to be on the classpath of your application in order to use
 the Spring Framework's email library:
 
-* The https://eclipse-ee4j.github.io/mail/[JavaMail] library
+* The https://eclipse-ee4j.github.io/mail/[JavaMail / Jakarta Mail 1.6] library
 
 This library is freely available on the web -- for example, in Maven Central as
-`com.sun.mail:jakarta.mail`.
+`com.sun.mail:jakarta.mail`. Please make sure to use the latest 1.6.x version
+rather than Jakarta Mail 2.0 (which comes with a different package namespace).
 ****
 
 The Spring Framework provides a helpful utility library for sending email that shields