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We have two servers with Merak Mail Server, serving a considerable number of accounts (15000). We had a terrible problem with our storage server, and we've been forced to move the mails to other server.

We don't know why, but email clients (Outlook, Thunderbird) are downloading again all the mails that were stored. Is there any way to force the clients to download only from now?

I've seen the option in GMAIL, but I haven't seen comercial package with that option.

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up vote 4 down vote accepted

my guess is that when you moved the messages the UIDL on the messages have changed. In POP what the client has seen is stored on the client, and it is done by UIDL. So if they have changed, then all of the clients are going to see the messages as new, and there isn't that much that you can do about it. (Save figure out how to make all the UIDLs of the messages match what they where pre move).

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Does the new server have a different name and IP address? If so, then the clients don't believe it is the same server as before, and therefore, the email it contains aren't the same ones it already fetched.

It is entirely up to the client whether to delete old mail from the server once it has fetched it, or to leave it on the server. If it is configured to leave the mail on the server, then it avoids fetching it again next time by keeping track of the mail UUIDs. If you configure the client to fetch mail from a different server, it throws out that old list of previously fetched UUIDs.

Leaving the mail on the server with POP3 has several problems and so is not recommended. If your users want to keep mail on the server, you should switch to IMAP instead.

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POP clients don't track emails by way of the server name or ip address. They track them by way of a unique identifier, specific to each message, which is usually the UIDL (although some may use TOP or LAST, but I doubt any do). – joeqwerty Sep 13 '11 at 18:22
@joeqwerty, the UIDL is specific to the server that generated it, so new server, new UIDL. – psusi Sep 13 '11 at 20:04
Right, but it's relevant to the MTA that the server is running (and how it generates UIDL's), not the name and/or ip address of the server. – joeqwerty Sep 13 '11 at 21:14
@joeqwerty, the MTA can not assume that a different server ( one with a different IP address or hostname ) uses the same UIDL, therefore it has to discard the old UIDL when you change servers. In other words, two different servers can have different messages on them that happen to have the same UIDL, but as far as the MTA is concerned, they are not the same message. – psusi Sep 14 '11 at 0:10
I hear you and I think I agree with you if I'm understanding what you're saying. What I'm saying is that your statement in your answer that the issue is being caused because the server name and ip address are different is wrong. The issue is caused because the new server has created a new UIDL for the messages. Which is what I think you're saying, right? Also, when you say "MTA" you are referring to the server, correct? – joeqwerty Sep 14 '11 at 0:52

As Doon stated in his answer, the most likely cause is that the UIDL has changed due to the fact that the messages have been moved to a new server. I'm certain the server would assign it's own new UIDL to each message and as a result, the client believes (rightfully) that these are new messages and proceeds to download them all again. I don't see any way to avoid this with the way that POP is implemented other than for the client to use TOP, which may not be implemented in the client.

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