Refer to the attached site list to see which services are offered at individual sites. Some services have gone offline, resulting in loss of mail. This applies to some smaller, specialized sites. Some user's report lags of up to a week. Test the delay when evaluating sites.
Many services have limits based on file sizes, excessive activity or inactivity.
Check their policies. You cannot access your received email when you are offline, unless saved to a disk, a local drive or the cache. There is no central directory listing, as each Internet Provider manages its own user records. Therefore it is much easier to identify a user name if you know their Internet Provider or host organization. There are a number of people-finding tools on the Internet.