get-mailbox | where {$_.WindowsEmailAddress -eq "user@domain.com"} | Add-MailboxPermission -User "user_who_need_access" -AccessRights FullAccess -InheritanceType All

or

Add-MailboxPermission -Identity user@domain.com -User "user_who_need_access" -AccessRights FullAccess -InheritanceType All
get-mailbox | where {$_.WindowsEmailAddress -eq "user@domain.com"} | Add-ADPermission -User "user_who_need_access" -ExtendedRights "Send As"

or

Add-ADPermission -Identity "user@domain.com" -User "user_who_need_access" -ExtendedRights "Send As"