How to check whether E-Mail was successfully sent in Liferay 6.1.20EE via MailServiceUtil

basically there are two problems which prevent you to discover if any failure occurred when sending e-mails with MailServiceUtil service.

Firstly messages are sent asynchronously over Liferay message bus, so the user gets faster response but you never know if anything failed on the way.

Secondly messages eventually get to MailEngine where all exceptions get caught and are only written to the log.

So, I suggest you to override MailEngine class with ext-plugin (it's in util-java) to handle exceptions differently and then use it directly from your plugin instead of MailServiceUtil which only pushes all to the message bus.