Hello Westwoodbay,
To help us get a better understanding of the issue, we’d like to ask a few questions.
1. Is this happening in the Apple Mail app, or another application?
2. What happens when you try to delete that email? Do you receive an error message?
3. If you do get an error message, what does the error say?
Please also check out the information in this link, to ensure the correct process is being used to delete that email, if you're using the Mail app: Delete emails on your iPhone or iPad
Swipe to delete and other ways to discard one email
• Swipe left quickly over a single email.
• Touch and hold an email from your Inbox, then tap Trash Message from the menu.
• Tap the Archive button
or the Trash button
from within an email.
You can also use the steps to delete multiple emails to delete a single email.
Thanks!