When you are in a list which has multiple pages (e.g. 100 mails with 10 mails per page) and select one of these to view the message there are a previous and a next button to select the previous and next mails. As soon as you reach the first mail of the pager you where viewing in the list before, the previous button is not available ("greyed out") even as there are more mails on the previous page.
The same applies to the next button when you reach the bottom of the list.
According to our users this bug did not exist in the previous release of squirrelmail.
We are currently using version 1.4.22