Previously I tried to compose a mail to a specific callsign. Here is what I did:
I was NOT connected, clicked "SEND VMAIL" and entered in lowercase in the "To:" field the callsign of a friend of mine - instead of that call which was already written there in the "To: - field" ) . I composed the message-text and saved the vmail to the outbox.
Later I connected my friend and my expectation was that the mail would be transmitted. But it wasn´t transferred.
I deleted the mail, recomposed it (my friend´s call already was there now because we had a recent connection) and then it worked.