It means the article ID specified in the RETR command no longer exists
up on the server.
When your e-mail client connects to the mail server, it will issue LIST
and UIDL commands. LIST lists them by a relative indexing number. The
server usually assigned unique identifiers (UIDs) to each article so the
UIDL command lists those. Indexing can get screwed up (out of sync) and
why it's better to use UIDs. If you issue the UIDL command but the
article is deleted at the server by another client or using the webmail
UI before your client uses the UID to RETRieve the article then it's not
there to retrieve anymore.
Use the webmail UI to your account and move all items out of the Inbox
folder into some other holding folder. See if your e-mail client works
after that to access your account for new incoming e-mails.
By the way, do NOT use the Inbox folder to store old e-mails. Move them
into a different folder. The constant re-indexing required with
hundreds or thousands of articles in the same incoming folder can cause
indexing problems. The Inbox is not intended for permanent storage.
It is also possible a message is corrupted up on the server. When the
client requests the message, the server cannot provide it. Something
about the message screws up delivery. You could use the webmail UI to
look at messages sitting in your Inbox. Look at the biggest ones,
especially those with attachments. Move them into a holding folder and
then see if your e-mail client can retrieve the other messages okay.
You may end having to move all the messages from the Inbox folder into a
holding folder because even a small message can get corrupted that
causes the server to not know how to process it for external delivery.
You could just read all your Inbox messages and then delete them so the
Inbox is empty and check if your client works okay on new incoming
e-mails to your account.