If I understand you right, you want to be able to send emails from two different addresses on the same domain. Something along the lines of:
[email protected] and [email protected]
You don’t actually need to log into two different SMTP accounts to do the above.
You could for example create [email protected] and use that to send ALL of your emails.
But in the Reply To: field, you set [email protected] and [email protected]
When the email is validated at the receivers end, it’s done using the @yourDomain.com part and not by the actual address.
So long as the domains match in the From: (smtp@) and ReplyTo: (sales@ or general@) then you won’t fall foul of any anti-spam measures.
What I think Yehudah was thinking that you wanted to do was have the ability to log into [email protected] and [email protected] which he is right, would need a heck of a lot of customisation to achieve.
So long as the different addresses you want to use are on the same domain (as you stated in your original post), then doing as I suggest will work. (I do it all the time).