Hi chesMELN, I'm Jayzar and I am happy to help.
Once the message has been marked as unread, you can view it as a new message and respond. However, you must remember that a user can still see that you have read the message even if you marked it as unread.
After marking a message as unread, the message is still marked as “seen.” The sender knows you have seen the message and may expect a response in return. If you do decide to mark the message as unread, it will only affect your end of the chat. You can leave the message without opening it; you are only obliged to respond after the message is marked as “seen” on the sender’s inbox.
Source: https://businesstechplanet.com/what-happens-whe...
and
https://businesstechplanet.com/if-you-mark-as-u...
Let me know if I can help you further!
Best,
Jayzar