Unfortunately this only works for multiple addresses with the same SMTP account on the same server. All mails you send will be from that account (check the MAIL-FROM field in the SMTP session). If you have separate personal/work accounts, you are stuck.

Gmail allows you to access external STMP servers with dedicated accounts. This allows you to basically use Gmail as a client to your corporate mail server. I've been using Gmail to consolidate all my mail accounts in different servers. It's really nice.

Oh, and threading “just works” in Gmail.