This is what I would do:

Select the e-mails you want in the first category. There's a pulldown menu labeled "More Actions". (next to the DELETE button) Choose New Label. Put in a label like "in e-mails". Repeat for "out e-mails". To views the e-mails in each category, go to the label links on the left column. The will be sorted by date, so it should be easy to what e-mails are from what month. Another way to sort months is that you simply make different labels--"January in e-mails" etc., or deselect the old e-mails as you're finished with them