"Send Mail From" Address

Because of the way mail headers work mail programs will send as the address that were sent to, and only default new mail to the account you pick.

Reason being that the mail address is in the replyTo: in the message header, and Say I have an Exchange account @mycompany.com and an @gmail.com account, in order to send that gmail out my @company.com address I have to tell my mail server to accept authentication/mail from an address that's not that mail servers domain. That, for a mail admin is a huge security hole, therefore it isn't allowed in most cases. Besides, if I do set it up to send a message from @gmail.com from my @company.com account when it reaches it's destination and hits their transport filters it probably will get rejected because my @company.com is not a verifiable sender for @gmail.com addresses.

Those are just examples, but it describes the gist of it. And why it will not work in the hugest majority of cases so mail programs don't even try - because trying to support that mail scenario is a support niitemare.