One of the most common Exchange admin tasks is setting mailbox delegations. Personally, I usually do this through EAC but once you're familiar with the Powershell counterparts, it can be quicker to use that instead. I've created this post as a reference to do just that.

In the reference below, the user "Ellen Somebody" requires access to the mailbox of "John Doe".

Send on Behalf

Set-Mailbox john.doe -GrantSendOnBehalfTo ellen.somebody

Send As

Add-ADPermission john.doe -ExtendedRights Send-As -user ellen.somebody

Full Access

Add-MailboxPermission -Identity john.doe -User ellen.somebody -AccessRights FullAccess -InheritanceType All