You should be able to see why emails are being rejected by Discourse. Click on the hamburger (top right) to access the Admin Settings page, select Emails and then chose the “rejected” tab. That will show why it is rejecting incoming topics emails.
I set this up for the first time yesterday, here are a couple tips based on what I learned:
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Ensure the email sending the new topic is using an email address for a user that has appropriate permissions to post a new topic by email. (Trust Level).
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Ensure you have set-up a custom email address for the category. You do this by changing the category settings. This email address needs to be the replies email address you created on gmail a unique +NAME for the category, so, for example it should look like: replies.forum+UNIQUE NAME FOR CATEGORY[at]gmailcom [where replies.forum[at]@gmail.com is the account you created and setup in the POP3 posting)
If your problem is related to the reply to a post by email, you can only do this, I think, when replying to a notification of a post via email. So, your test account needs to have received an email from your forum and then you reply to that email.