I check 3 POP mailboxes with the xfce4-mailwatch-plugin.
From time to time it stops checking for new mail with no
indication of a problem and I have to log out and log in
again to restart the checks.
It may well be that others are not affected by this
problem - I have been logged in for 10 days and the
problem occurred after 7 days

However – it is impossible to rely on this tool.