Hello RichardPatterson,
If you have multiple accounts signed in, you'll see multiple Inbox and Sent folders under the parent folder. For example, if you have two accounts signed in you'll see:
Inbox ⌄
Account 1
Account 2
To properly troubleshoot the issue we need to narrow down where it's happening. Currently, the issue could be stemming from your Mac or from the email provider. One really easy way to see where the issue is stemming from is to open webmail for your account and send an email to the same account. This tests the send and receive functions of the account. After you receive the email, while still on the webmail site, check to see if shows in the correct place. If it does, the issue is related to the Mail app on your Mac. If it does not, the issue is related to the email provider.
If the issue is related to the Mail app, we recommend reaching out to Apple Support for more help: Get Support They should be able to screen share and help you with this.
If it's related to the email provider, we recommend reaching out to them for help.
Thanks.