In the IMAP standard, when you "delete" an item, it isn't actually deleted permanently. Instead, it's simply marked as "deleted". This is often displayed in email clients as a crossed-out entry in your mailbox. At this stage, the email still exists in your mailbox, it's merely been flagged for deletion. To actually delete the item permanently, you have to "purge" the mailbox. Most IMAP clients have an option in one of the menus or on the toolbar called "purge", which permanently deletes the email.
Most people find this system counterintuitive, so almost all email clients these days don't "mark messages as deleted", but instead just move the email to your Trash folder. When you log into our web interface, we will automatically purge any emails marked for deletion, so you will never see a "crossed-out" email. If you see this in an email client, there might be a setting to change the behaviour, otherwise you will need to update to a newer app, or use our web interface instead.