How can I remove attachments from messages without removing the message itself?
This is currently not supported. As a workaround, move the message to the drafts folder, double click on it in order to open it in the composer, remove the attachments, save the message again to the drafts folder, move it back to its folder. The disadvantage of this workaround is that the date will be changed to the current date. Some other headers might also be changed.
