We disabled a user's mailbox due to them leaving the organization. How do we make sure that we have all the mail from the mailbox in Retain?
Retain cannot enter a disabled mailbox in Exchange. The mailbox needs to be re-enabled and for security purposes the password changed so no one else can enter it.
If you haven't already, create an Archive Everything Profile that archives all items.
Then create an Archive Everything Job with the Archive Everything Profile and a single Schedule against just that one user. Start that job manually.
Once the job is complete, you may disable the mailbox.