The Confusing Case of Cross-Forest Delegation

If you've even participated in an Exchange Online migration at almost any level, it's likely you've run into the issue of cross-forest delegation. You know that Exchange allows you to delegate rights from one mailbox to another, allowing users to access other mailboxes. When you do an Exchange hybrid migration, there are some special considerations you have to take to keep these delegated rights working. Depending on who you ask, you'll get all kinds of different answers about what works when. In this blog post I will explain the confusing case of cross-forest delegation, and what you can expect to work or not work.