You would have found the answer fairly quickly with a quick search of the forums. The feature you need to change the sender 'from' address is in Preferences/Accounts. To enable the polling of external mail services you need to do this:
'default' is the COS and '60' is the number of seconds for polling, set it to whatever you like and it can also be specified in minutes e.g. 5m.
Check the setting with:
zmprov gc default | grep zimbraDataSourcePollingInterval
enable it with:
zmprov mc default zimbraDataSourcePollingInterval=60