Depending whether you have logged in using the login screen for your email mailbox from the browser or from MailCheck, you may be logged out of MailCheck after a period of inactivity for reasons of security. This is normal behavior for email and is for your security.
You have stored your login data in MailCheck, but have been requested to login.
Your login using MailCheck will be reset after several minutes for reasons of security. The letter icon for the inbox will be grayed out or assigned a red not-logged-on icon.