ANSWER:

The best way to understand this is to try to send mail yourself using different methods.

I have found that the UID changes based on two factors (but there may be more).

1. If the script uses the sendmail binary then it will reflect the UID of the user that invoked the script.
2. If the script directly connects using SMTP it’ll use the UID of the MTA.

J Moore from http://unix.stackexchange.com/questions/76421