send() now returns the message id of the underlying MimeMessage

Sometimes it's good to have this information so you can store the message id e.g. in a database and can look it up later in case the mailserver had problems sending the email or e.g. when a bounced message comes back.

See http://commons.apache.org/proper/commons-email/javadocs/api-release/org/apache/commons/mail/Email.html#send()