I set up the Gmail account through BIS manually and the same problem is still there. I made sure all the settings were correct (port 993, etc.) and Gmail proved more of a problem than Yahoo - In addition to the sent message you can't delete from the BB there is an "auto confirmation" that goes in the inbox telling you that the message was delivered... Gmail won't delete that either (Yahoo will) - you have to do it from the computer or from Gmail via the browser.
In the end I don't have a problem with deleting sent messages from the computer as long as they are in Outlook. Since Yahoo does not have IMAP through Outlook I have to access Yahoo from my computer browser to delete the sent messages I send from the BB... meaning I am "managing" my Yahoo mail from three locations which is flat out unacceptable.
Sometimes I wonder if the "tech" things in my life are only here to make life more difficult for me... the utterly tragic part is I pay money for this... I think they should pay me for "pain and suffering"...