By default, when Mulberry opens a mailbox it only downloads the message captions it needs to fill the current display area in the mailbox window. As you scroll, more captions are downloaded from the server as needed to fill the new areas of the display. This ensures that opening a mailbox is fast as the amount of data transferred over the network is small, opening a 10,000 message mailbox takes basically as long as opening a mailbox with 20 messages in it.
However, when the mailbox is sorted by a category other than the message number (which is the 'natural' sort order of the mailbox) all the message captions are required in order to calculate the sorted positions of the messages in the mailbox display. As a result Mulberry has to download all the message captions from the server. Before doing that Mulberry pops up the 'Full Sort' dialog which gives you the option of halting the sort and going back to message number sort which doesn't require a full caption download.
The threshold at which the full sort dialog is triggered is controlled by the 'Sort Warning' preference in the Mailbox>Advanced preferences panel in Mulberry. If the number of messages exceeds that limit, the warning will appear.
To prevent the alert from appearing, you can do several things:
- Switch sorting back to message number
- Reduce the number of messages in the mailbox by moving some to another mailbox, or by deleting and expunging unwanted messages
- Increase the 'Sort Warning' threshold in the preferences, but be aware that this may cause longer opening times when opening a mailbox.
Alternatively you can just live with it!
NB Some IMAP servers support a new experimental server-side sort function. Mulberry can make use of this capability, and as a result doesn't have to download all the message captions to do a sort. For such servers, the 'Full Sort' alert will never appear, provided the sort category is one that is supported by the server.