In addition to the outgoing email address, it need to know which SMTP server to connect to when sending.

actually no it doesn't as most people use an ISP. In a corp setting perhaps this would be required to validate proper sender.
But I have yet to have a problem when I send from Adelphia/Comcast to anyone from any of my email domains.
In my case I can only use one SMTP outgoing, my ISP, when I try through other locations, aside from "send from Domino server" it would fail.
In my accounts setup I have the SMTP listed for all users usage.

Would you like your users to use a name that could have authority, to generate and send email?
for example, in outlook, i could change my sending name to the president of the company, and send a note to accounting to send money or give me a raise.
the whole purpose of notes as a "secure email system" is to authenticate senders. I think security trumps ease of use for this scenario.

The location document email address field can be used to override the PAB email address if the administrator allows it. So you could keep the id/mail file, and just switch locations to use a different external mail sender address.

This is a nice add-on in N8.5.1 you get to select the address for a user. Demo'd it for a now former Outlook user who exclaimed that Notes couldn't do this and *poof* there it is. She liked the implementation.

For clarity, when you type an address in an e-mail, move your cursor down the list to select the address from recent contacts and contacts that type-ahead presents. if the target provides more than one address, you will see a sub-list extend to the side and show the address and label. Just select the alternate/preferred address.

Also, if you hover the cursor over any address in the list, you will see a gray pop-up list with two nested downward pointing arrows. Click on the arrows to see all of the addresses for your user.

This capability is on by default, iirc.

If you have any questions, please ask.

This request was not about the recipient's address, it was about the sender's.

Alan beat me to it. And I am happy to see so many people agree with me on this one

@6 Obviously, I read the title differently...as the solution I provided addresses a common problem end-users face, as you know. Thanks for input.

@0 / @2 The Notes security model would come into effect here, as it would then be too easy to just drop a name and stamp it, unless it was a valid address. Yes, you can do this with some 3rd-party mail clients, and I had no problem putting false from mail addresses (innocuous addresses, I avoid nefarious activities for the record) and having them route out and being stamped from that false address. So, for this to work, there would need to be a security verification enhancement.

Might be easier to allow multiple addresses for the same Notes Domain, to assist AA's and Admins with bulk e-mail addresses. They would have both IDs and can easily be verified with a PNAB template enhancement in the location document.

Additionally, several Tier1 ISPs block routing mail from a return address that is not on their domain. So, you would in-fact need @1 's suggestion of an outgoing server, password and login (if they require authentication).