To talk about a past event that is recent you can use HAS, it focus on the recent past.
Let´s now anticipate your next likely question:
Why would "a message has sent..." be in a different sense of "a message was sent..." ?
Which would lead us to "ILOVEBRITONS sent
you a new message on InterPals." (it could be possible such a thing, it would be grammatically correct, I mean.)
In "a message has sent..." it takes into account that the e-mail program is so quick that it sent you the e-mail and then it notifies that "it has just sent..." (of course, it´s programmed to do so, it doesn´t study grammar on the English Experts). https://english.stackexchange.com/quest ... s-has-been