If the project is closed, Project owners can send messages as long as the Questions Due Date is set to Never or set to a date after the project's close deadline. The Opportunity Q&A functions the same, even after the close date has passed.
For more information about setting up the Opportunity Q&A feature, please see our Opportunity Q&A article.
Vendors can submit messages via the Opportunity Q&A passed the project's close date as long as the Questions Due Date is set to Never or set to a date after the project's close deadline.
If you are a Vendor and would like to use the Opportunity Q&A system to reach out to the purchaser, please see our How do I contact the Project Owner? guide.