Is it possible to change the from address on Mac OS X server email alerts?
I know this question is a few years old, but I had the same issue some up today. I resolved it with help from this post:

https://www.cyberciti.biz/tips/howto-postfix-masquerade-change-email-mail-address.html

This was on a Mac server that did not have mail enabled. Apparently even with postfix disabled, the alerts function fires it up just long enough to send the alert email and then closes it again. Running nettop while sending the test alert email lets you see sendmail kick off, followed by postdrop, master, pickup and qmgr. 

I found five different locations for the main.cf file file, but the one that seemed to be active was located in:

/Library/Server/Mail/Config/postfix

Edit that file to append this line:

smtp_generic_maps = hash:/Library/Server/Mail/Config/postfix/generic

Then edit generic by appending the mapping you want:

alerts@server.local alerts@validdomain.com
Then run:

postmap /Library/Server/Mail/Config/postfix/generic