The problem: I have a few users complaining almost daily that sending messages is extremely slow with their IMAP clients. They will hit send sometimes and the message will take between 20-seconds and forever (aka, it hangs) to send some messages. Other times the messages go out in just a few seconds. This seems to be much worse with Thunderbird, but still happens with the Apple client.

Here's a bit of background...

We recently switched from a hand-built pair of linux-HA'd mail VM images (dovecot, postfix, amavisd, spamassasin) storing active mail queue's on VMFS disks (over NFS from netapp filer). user maildirs were mounted via nfs. With this setup I never had any performance complaints, and average vmware server load was around .8-1.2 with 2 vcpus and a gig of ram. all accounts were managed via ldap.

Our new setup is a single VM image (4 vcpus, 4GB ram, VMFS volume mounted from a NetApp via NFS). Running the latest Zimbra OpenSource edition, all services on this single image. zimbra ldap, amavis, spamassassin, web ui, etc all running. Currently the Zimbra filesystem is a separate VMFS disk image (again, vmware mounts these via nfs) running ext3. The zimbra message store is CURRENTLY a direct netapp nfs mount with about 35GB of messages.

Generally the vm image seems to have a load of between .4 and 1.1 depending on how many messages are coming in at any given moment... we have ~50 users and receive 20-30k real messages per day (not including spam thats blocked before it gets into the system). Only 2-3 users use the web interface, the vast majority use IMAP clients. All of the IMAP clients are using IMAP SSL (993) and we are using SSL on ports 465 and 587 for authenticated outbound SMTP.

*** i am aware that NFS is a slow mailstore back end. I have built an iSCSI lun and synced the messages over, but I need to wait until tonight to do the switchover in Zimbra. ***

I really could use some recommendations on how to troubleshoot this... I just don't really see these slow mail sends myself, and neither do most of our users. The two users who are complaining the most each have the most amount of mail in their accounts (3.5GB and 5GB), but there are quite a few other users who are above 2GB.