Occasionally After a new RMail installation, the "Send Registered" button doesn't appear  when composing a new email.  If you go to the Outlook File menu and then check Options and find that Rmail is not on the list, but it's showing up on the add-ins list, try the following advanced troubleshooting steps -

Warning, editing anything in your registry can cause serious system errors  

Keep in mind, you might not find all folder locations listed below, depending on the user’s system: If you do not see one of these folder locations, just skip it and move onto the next step. 

1. Close Outlook. Open regedit

2. In the Registry, follow the path pictured below:
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3. Modify the LoadBehavior to set the value to 3

4. Do the same in steps 1 -3 for this path as well 
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*do the same in HKEY_Current_User\Wow6432Node….

5. Close regedit. 

6. Remove the file "RMailOffice.settings" from the file path C:\Users\(your)Username\AppData\Roaming\RMail\RMailOffice

7. Open Outlook. The button should now automatically load up. If it doesn’t Go about the Outlook steps to make the RPost button active within FILE> OPTIONS>ADD INS. Then reboot again.