This was built specifically for my needs but it'll work for any mh mailbox pending you know what to do...

mailarch script

Partial output from my run... (I only started using NMH in July)

fury(~) [34] > ./bin/mailarch
.archive+ has 3057 messages  (1-3469); cur=143; (others).
Jan.2003.tar:  scanning... no email this month.
Feb.2003.tar:  scanning... no email this month.
Mar.2003.tar:  scanning... no email this month.
Apr.2003.tar:  scanning... no email this month.
May.2003.tar:  scanning... no email this month.
Jun.2003.tar:  scanning... no email this month.
Jul.2003.tar:  scanning... tarring... bzipping... -  69 mail
Aug.2003.tar:  scanning... tarring... bzipping... -  140 mail
Sep.2003.tar:  scanning... tarring... bzipping... -  27 mail
Oct.2003.tar:  scanning... tarring... bzipping... -  246 mail
Nov.2003.tar:  scanning... tarring... bzipping... -  25 mail
Dec.2003.tar:  scanning... tarring... bzipping... -  189 mail
Jan.2004.tar:  scanning... tarring... bzipping... -  617 mail
Feb.2004.tar:  scanning... tarring... bzipping... -  811 mail
Mar.2004.tar:  scanning... tarring... bzipping... -  932 mail
Apr.2004.tar:  scanning... no email this month.
May.2004.tar:  scanning... no email this month.
Jun.2004.tar:  scanning... no email this month.
Jul.2004.tar:  scanning... no email this month.
Aug.2004.tar:  scanning... no email this month.
Sep.2004.tar:  scanning... no email this month.
Oct.2004.tar:  scanning... no email this month.
Nov.2004.tar:  scanning... no email this month.
Dec.2004.tar:  scanning... no email this month.