This issue mostly happened after users added their email account from comcast.net, Gmail and AOL to iPhone. When they deleted emails from iPhone, they keep coming back.
So how to fix this issue?
Are you use same account on the another device like MacBook? Try to check preferences on your MacBook Pro or iMac and see how it is handling deleted messages.
- Try to change your system preference under Accounts -> Advanced -> Remove copy from the server after retrieving a message and change it to “immediately”.
If you are using Gmail, go to the SPAM folder and delete all of them forever.
Reboot your iPhone and try to delete it.
Delete archived messages in your Comcast account.
If your problem not solved yet, do let us via comment.