Just to let you know that you are not being ignored :) When someone does not respond it usually means they do not know an answer. I've tried (unsuccessfully) to find some info on this. Note: the number of lines is not really the issue because if you expand or contract the width of the window you will notice a change in number of lines used, but the actual content remains the same. This is probably more to do with set limits on KB size / number of characters. Perhaps because the portions displayed are more like a search result so may also be linked to the indexing file rather than the mbox file. As a lot of messages could potentially be selected, it would not be unreasonable to think some restriction is inevitable. Thunderbird should not list more than 500 conversations in the message pane even if more than 500 are selected. So, it sounds like any given message will be allowed x amount of space.