Some mails from different accounts can not be deleted. All accounts are IMAP. Some thread says that switching off the conversation view would help, I can not confirm.
Thank you
If you have IMAP account emails that cannot be deleted due to whatever reason, close eM Client and go to your IMAP (Webmail) GUI and delete those emails there.
Then re-open eM Client and click “Refresh” at the top left and those deleted mail will be gone in eM Client. Just sounds like some mail sync problem, so the above will normally fix that.
Note:- If you delete the emails online ok, but closing and reopening eM Client and refreshing still doesn’t fix it, then you will either have to remove and re-add those problem IMAP accounts or make a backup of eM Client via “Menu / Backup” and then do an eM Client restore via “Menu / File / Restore” to restore and older version of the email database where everything was working ok.