The message store has a hard limit of two gigabytes for a store.idx file, which is equivalent to about one million messages in a single mailbox (folder). If a mailbox grows to the point that the store.idx file will attempt to exceed two gigabytes, the user will stop receiving any new email. In addition, other processes that handle that mailbox, such as imapd, popd, mshttpd, could also experience degraded performance.
If this problem arises, you will see errors in mail.log_current such as this:
05-Oct-2005 16:09:09.63 ims-ms Q 7 ... System I/O error. Administrator, check server log for details. System I/O error.
In addition, the MTA log file will have an errors such as this:
[05/Oct/2005:16:09:09 +0900] jmail ims_master: Store Error: Unable to append cache for user/admin: File too large
You can determine this problem conclusively by looking at the file in the user's message store directory, or by looking in the imta log file to see a more detailed message.
The immediate action is to reduce the size of the file. Either delete some mail, or move some of it to another mailbox. You could also use mboxutil -r to rename the folder out of the way, or mboxutil -d to delete the folder (see 188.8.131.52 The mboxutil Utility.
Long-term, you will need to inform the user of mailbox size limitations, implement an aging policy (see 20.9 To Set the Automatic Message Removal (Expire and Purge) Feature), a quota policy (see 20.8 About Message Store Quotas), set a mail box limit by setting local.store.maxmessages(see configutil Parameters in Sun Java System Messaging Server 6.3 Administration Reference), set up an archiving system, or do something to keep the mailbox size under control.