Choosing sender email address

I need to send out emails to a particular company with a sender email address that is different than my default email address.  I set up two different pop and smtp records in the 'Internet options...' for my default email and the other email address.  I thought that if I put the same names in the Identities section, that all I would have to do is select the pop/smtp name I wanted, but is doesn't work that way.  Anyone know how to choose from several created pop/smtp internet option?  Thanks