For POP3
The mail clients (this is outlooks default behavior) will send a delete 
command to delete all of the messages on the server for their account after 
the client has finished downloading the messages. In the client this sending 
of the "delete" message can be disabled or modified to delete after certain 
days or other possibilities. The delete command will also not get sent if the 
connection to the pop server is broken prior to downloading all of the 
messages. Next time they check for new mail they will get all of their new 
mail again.

If there is a successful download of all messages and the client is NOT 
deleting the messages after download the mail client will remember which 
messages it has already downloaded from the server. If they change the name 
of the pop3 server (even it is to the same server just under a different 
name) the client will think this is a new server and download all of the 
messages.

In any case, unless there is some good reason to keep the messages store on 
the server with pop3 accounts, don't do it.

In outlook (mostly applies to outlook express too) the setting to modify this 
behavior is located in

Tools > E-mail Accounts > "Change" the account in question > Advanced tab > 
check boxes at the bottom.

If they are getting this behavior with IMAP I am not sure what could be going 
on. Make sure they do not have their account setup twice in the mail client 
or something I guess. 

Does everyone get dupes or just certain people?

On Wednesday 12 October 2005 09:00 pm, Josh Welch wrote:
> Raymond Norton wrote:
> >> I expect they're probably
> >> downloading the same message over and over again with POP, which would
> >> be a whole different problem.
> >
> > We are using dovecot for Imap and pop3. Is there anything that can be
> > configured to prevent the duplicates? In the past I have found if a
> > user's pop3 server was set up as mail.domain.com, and later changed to an
> > IP, all messages that are stored on the server would come down again.
> > Nothing has been changed, so I'm not sure why the problem has cropped up.
>
> The last thing that a POP3 client does after downloading email is to
> tell the server, I got this email, don't send it to me again. If your
> connection to the POP3 server gets terminated prior to the client
> closing the connection gracefully, they will download the same messages
> again next time they connect to the server. This behavior can be caused
> by network issues or POP3 clients behaving poorly.
>
> Josh
>
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