I would start with pulling a bounceback report on your user marked as invalid (Email Bounceback History with Messages report) and looking at the Bounceback Message and SMTP reply error code you are seeing for this contact. This will help you determine what action needs to be taken.
Also, a good idea to work with your IT team to whitelist your Eloqua domain/IP as well as have contacts add you to their safe senders' list.
Note the following KB articles that can explain why you may see a soft - hard conversion on bounceback status, which would mark your contact as invalid:
Document 2255905.1 (Deliverability: Will a Soft Bounce Ever Convert to a Hard Bounce in Eloqua?) can be found at: https://mosemp.us.oracle.com/epmos/faces/DocumentDisplay?id=2255905.1
Hope this helps!