Overrides From set in code with site email.

Problem/Motivation

I have code to manually send various emails from our site. We set different From addresses in the mail header depending on the email we send. We have been using Mimemail (and for test, also php mail) senders for sending email and this has always worked fine. We recently had to switch to using SMTP for sending email and just noticed that we are no longer able to set the From address as SMTP (when the smpt config From is not set) always uses the site email address (as opposed to checking first if it is already set).

Steps to reproduce

Proposed resolution

SMTP should not set the From address if it is already set. Possibly if the SMTP From is set then it makes sense; but if it isn't set, it shouldnt be automatically setting it to the site email.

Remaining tasks

User interface changes

API changes

Data model changes