If you use right click on an unread email in the list and select 'Delete Message', it will be put in the 'Deleted' / 'Trash' folder and marked as read. Using this method means it will not be opened in the Message Pane regardless of whether it is enabled or not. if Message Pane is open a single left click on email in list will open the email in the Message Pane. However, if you delete an unread email from list, then you may discover the next unread email will open automatically in the Message Pane. If you do not want this to occur there is an addon extension called 'deselect on delete' which can help you. * https://addons.mozilla.org/en-gb/thunderbird/addon/deselect-on-delete/ I have located some info regarding the auto marking as read of unread deleted emails. Someone posted it as a bug request. I suggest you add some info such as version of Thunderbird and what OS you use. Request a means of disabling this and vote for the bug. Voting located near the top - Importance: -- enhancement (vote) you will need to register to post comments. Please note the link is not a chat or help forum, it is used by the developers, so keep comments relevant with useful information. * Bugzilla : https://bugzilla.mozilla.org/show_bug.cgi?id=545504