Outlook 2010 should have a Work Offline button on the Send / Receive tab, but this is missing. To get the Work Offline button back:
- Open Outlook 2010
- Right click anywhere on the ribbon
- Click Customize the Ribbon…
- Click New Tab
- Select Work Offline from the left hand side list
- Click Add
- Click OK
The Work Offline button will still be missing from the Send / Receive tab, but you will now have a new tab (called New Tab, unless you renamed it) that has a Work Offline button.
❗ Addendum: The Work Offline button can also be missing if Outlook is not set to Cached Mode. If cached mode is already on, you may need to recreate your Outlook profile.