In Recepient you can use action > mass update and add field timezone and select EST as value .. Update all will update the timezone at user level
For Server level timezone change you will have to insure below stuff if hybrid model ..
Solution :-
Revert back the oracle client to match the oracle database server version and restart. After that timezone will work as expected. however while investigating we did couple things -->
1. Changed the user timezone setting on MTA server
2. Changed the MTA server timezone
3. on marketing Adobe Campaign software side changed user timezone
4. on marketing Adobe Campaign software side downgrade the oracle client on app and web server to match the database version i.e. 12.1.0.2.0
5. Restarted all