While it is certainly possible that a Microsoft-managed network places additional restrictions on what addresses are used, there are several good reasons to not

While it is certainly possible that a Microsoft-managed network places additional restrictions on what addresses are used, there are several good reasons to not to create unique "snowflake" situations for individual applications. There might be silent assumptions everywhere what domain addresses answer to, in log collection, in monitoring, in address management tools. Simple things like domain search strings becomes harder. Any non trivial enterprise is going to have a ton of these (semi-)automated tools that your address will end up in. It also carries the risk of complicating future work. The company might merge with another company. Or imagine a mandate where all communication will need to switch to TLS, only to discover there are addresses used under other domains internally.