This is Outlook 2013 talking to an Exchange server. Though I think that is irrelevant, I think that this is Just The Way Outlook Handles Rules Period.
Outlook doesn't process rules in an Exchange setup, the Exchange server does, at least in default configurations and rules setups Period. Only when you check something to make it a client side rule, or go beyond the limits of Exchange rules does it truly turn into an Outlook rule and run through Outlook's rule parser.
The distinction bit me quite often when dealing with Exchange on a Mac. It's taken until Exchange 2010 SP1 for them to really start cleaning this mess up and enabling their Mac Office products to deal with Exchange rules.
Should be easy, right? I just make sure that, in outlook, when creating the rule, I make sure the filter is for "build@mycompany.com" instead of the group list "mycompany build mail". Right?
Doesn't work. No matter how carefully I type it into Outlook or OWA, it always always always converts it to the group list. I can't NOT convert it into a group list when I enter it. Sigh.
Yep, this is again due to Exchange handling the rule. It's been a while since I've poked at the internals of Exchange, but in the past it used it's own proprietary messaging format. Part of this is due to the age of the product, and existing in eras when not even TCP/IP was seen as the way forward, much less IMAP/POP/SMTP standards. Internet e-mail addresses are translated into whatever internal objects Exchange uses to represent destinations for the message. It sounds like in this case it's being fed "build@mycompany.com" and seeing an Exchange or Active Directory group with that set as it's internet e-mail address, and mapping to that group object.
*edit* I think I remember a weird workaround to avoid it turning the address into a group. But in searching for it, I found a cleaner workaround that may work. Look for the filter option of "with specific words in the senders address" instead of "from". Put the e-mail address in there instead.
I want to spambin messages from the build server, but not spambin messages from that group list.
Start looking at the raw headers for something else to filter off of. Most build systems I've used insert some sort of header into the e-mail that could be used in filters. To do this in Outlook 2013, you have to open the message in a new window (not the reading pane), go to File, Info, Properties, and look for the text area titled Internet Headers.