I'm having a bit of a problem when trying to migrate some mailboxes from an existing mailserver to the latest Zimbra Network Edition. I am using the migration tool located on the admin site.

Admin site->Left side, Tools box->Account Migration->Migration Wizard

During the migration, I'm getting the following error:

2011-10-03 10:05:50,110 INFO  [Thread-319] [] datasource - Requested import.
2011-10-03 10:05:50,110 INFO  [Thread-319] [] datasource - Importing data for data source 'imap-import-server'
2011-10-03 10:05:50,167 INFO  [Thread-319] [] datasource - Import completed for data source 'imap-import-server'
2011-10-03 10:05:50,167 ERROR [Thread-319] [] extensions - Error in IMAP import task for account 478e20c5-3381-4b8c-9802-2e86b2f14367, datasource 52ff8055-07ea-4e56-bc77-cdc4ea09a007
com.zimbra.common.service.ServiceException: system failure: Unable to connect to IMAP server: DataSource: { id=42ff8055-07ea-4e56-bc77-cdc4ea09a007, type=imap, isEnabled=true, name=imap-import-server, host=mail, port=143, connectionType=cleartext, username=zecuthi, folderId=1 }
Any idea what could cause this ?