Hello, susanmtbc.
Thank you for reaching out to Apple Support Communities.
From what I've read is that you're unable to permanently delete emails in the trash folder. I'd like to help you on this. If you open the trash folder, do you have the Edit button in the top right corner? If you see the Edit button, tap on that which then you can select email to delete, mark or move. Or are you able to swipe left to pull up additional options on one of the emails? These are two ways to delete them: Delete and recover emails in Mail on iPhone
Check to see if you have any archived options checked for your email. Also, does this happen with one email account or all email accounts?
See if you can delete them using a web browser by logging into the email providers website, if this is possible.
Keep me updated.
Cheers!