Office365 - Can I use my old email address to send emails after migrated?

No. You can no longer send message with using the old email address after migrated to @Connect. When you send message using the HKUST email server, the From field of that message will become <your account>@connect.ust.hk.

However, others can still address to your old email address (@stu.ust.hk/@ust.hk or full name email alias) as far as you are studying in HKUST. After you have finished your study in HKUST, people will need to address to your lifetime @Connect email address.