I would strongly recommend that you set up all the accounts in an email client, such as Outlook or Thunderbird (I use thunderbird as a personal preference - I think it is easier and more effective but this is just my opinion - use what you are comfortable with).
Then set up all the seperate accounts in your email client (In thunderbird you go to Settings --> Accounts --> Add Account). If you do this then you can select from a drop down menu which email address you want the email to be sent from.
This is hundreds time easier than what you seem to want to do and I would recommened this way to be used as it is by far the easier option and means you can still check all your emails in one place (the email client) from all email addresses added to it.
The system that you are currently using seems to be overcomplicated when it doesn't need to be.
Hope that helps
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