Hi @suanlian
Something in the email contents (subject or content) is getting flagged by your email server and the email gets put in the spam folder.
If you have access to the control panel (for the hosting) there should be a section about emails in there which will show all incoming emails and their status, for the ones that got spammed there should be additional information on what triggered them to get spammed.
If you’re not sure where to find that the best approach is to contact the hosting provider (unless the email service you use is external) and ask them to check what triggers it to go to spam.
Alternative is to make a filter (again, in the email service settings, not the plugin) so that emails that come from the form (for example filter based on email subject) do not get sent to spam.