Exchange: Application errors 1053 and 1056 (POP3-Connector)

Since one month we get in the events/application following errors:
Today: 06.Oct.2007 16:45:40: Error 1056 (POP3-Connector)
Code: -2147024864
Today: 06.Oct.2007 16:45:40: Error 1053 (POP3-Connector)

No other errors (system, and so on at the same time).

In many articles I found solutions:

1) One of the DNS-IPs of the provider have been changed: Solved
2) New Updates of Windows Server 2003 with SP2 installed
3) That this can be a spam mail and therefore it was not delivered
4) The size of the mail is too big, that I was not delivered

In Failed-Folder there is no mail with this ID.
I do not find any good explanation, was the cause of the error is in protocols.
I do not find this mail on the server.

Thanks for your help

It is possible, that this error entry only appears due to following situation:

- Win will POP3-Connect will receive all messages of all users from unix box
- In the same time for one account, there is an incoming mail an therefore the mailbox is locked.
- Win can not connect to this mailbox and the POP3-connector is very sensible. Therefore the POP3-Connector is producing an error message in event-logging.

Can this be the reason ?

Thanks Harald