Any set 'from' address is ignored when building the email body

Added logic for handling a given 'from' address

NB. this seemed to work previously, but looking at the old code, it was never supported, so I might be missing something

We were using 📌 Use 'From' email address for the sendmail POST user ID RTBC before, hence the working setup before updating, but that got reverted, so this is a new attempt at that but using the new helper method

So redid that patch to use the new helper method. Also redid the current logic for the default from address since the ?? doesn't do what the code hopes it does since default_mail will get set to an empty string.