Redownloading of old mails after migration

[TorbHo]

After server migration our clients had to redownload all old eMails again (POP3). So some of our clients had to download gigabytes of old mails.

We migrated two separate servers. Both of them had the same problem.

How can we prevent this problem for the next time, if we ever have to migrate Plesk again?

Old and new Server: Plesk 12.5.30, Ubuntu 14.04.3 LTS‬, Postfix and Dovecot

 

[xyptilon]

As explained on Microsoft: https://msdn.microsoft.com/en-us/library/office/dn352480.aspx, but applying to all POP3 accounts.

Messages for a POP account are identified by unique identifiers (UIDs). An email client that leaves mail on the server uses the UIDL command to retrieve the UIDL map that associates each message that has been delivered to the mailbox to its UID. The client also gets the UIDL history for messages that have been downloaded or deleted for the Inbox on that client. Based on the UIDL history, the client can determine which messages are new and should be downloaded.

So, if you move mail, and the new mail server generates UIDL lists differently then your mail program will think everything has to be downloaded again. To prevent this, always ensure the format of the UIDL is the same.

That being said, even if it is the same, sometimes, due to a hostname or IP change, the e-mail program will download everything as well.

 

[TorbHo]

Thank you.

The hostname and the ip were changed both.
Is there no possibility to migrate the server without re-downloading old mails via POP3?

 

[xyptilon]

Behaviour is different depending on the e-mail program. Even if you can keep the same hostname and IP address and even if the UIDL format remains the same, some programs will still download everything again.

To clients having large mailboxes POP3 is less than ideal (and very slow). Perhaps you can suggest them to switch to IMAP. If not, it's a one-time inconvenience for them.