On our server, we have two separate email systems: if it's a long queue of recipients, then we store it in a queue to be sent in a cron. If it's less than X number of recipients, then it sends immediately. Both are built in PHP with MySQL storing the queue.
However, there is a question of whether or not this is working 100%. In order to determine which email system is sending the email, I'd like to add a simple, completely meaningless email header like foo:bar or something like that.
So, my question is: is this at all possible? Would a foo:bar email header screw anything up? I'm not very well versed on what's under the hood, here and it's a live system sending out thousands of emails per hour.