When you set up an account, you specify a user name and password for it. If you connect to Hurricane Server via SMTP and authenticate with a user name and password matching those you have set up for an account, that is the account into which outbound messages will be injected.
The anonymous account does not have a user name and password because it does its authentication via Hurricane Server's white and black lists. All other accounts are controlled and selected by their user name and password via SMTP authentication.
If you need assistance with SMTP authentication on the client side, please consult the documentation that came with your development platform.