Document Created: 14/September/2020
Send As, Send on Behalf and Full Access for Exchange server 2010/2013 - https://social.technet.microsoft.com/Forums/office/en-US/26447ae6-26dc-4b78-9f2c-fb85cfdd6ae3/send-as-send-on-behalf-and-full-access-for-exchange-server-20102013?forum=exchangesvrgeneral
If using the Outlook Windows client and cannot send email as a shared mailbox. Switch off Cached Exchange mode. This will force Outlook to use the GAL permissions for send as instead of (likely) out of date cached local permissions.