We do not post messages to twitter that begin with @username in an effort to combat spam.
On Twitter and StockTwits (the only services this applies to), posts beginning with @username are directed to that specific user, and other users won't even see the posts unless they also follow that user. The only exception is if you use @yourusername, which all of your followers will see because they follow you and the "target user", which is also you.
If you are using a prefix that begins with '@' dlvr.it will not post your items:
As explained above, we do not allow feeds to automatically post to Twitter beginning with "@". If your feeds are configured so that they are prefixed with "@username". You will need to update those configurations to remove the @, or change the prefix to a suffix. Common methodology is to use a suffix of "/ via @username".
Also know that by using the prefix @username, only users who follow BOTH your account and @username will ever see the post. Twitter does not show messages beginning with @ to all users - only to those users who can see "both sides" of the conversation.