I recently had a situation where an email had a bunch of corrupt attachments. Each time one of the attachments was clicked on before the Mail app could mark the message as read the Preview of the attachment crashed the application. Turns out this was because opening the email automatically was attempting to display the image preview. Turns out I just needed to disable preview in mail long enough to get the messages deleted. To do so write the DisableInlineAttachmentViewing key as true into com.apple.mail as follows:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool true
To then disable this option, set the key to false:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false