We use dovecot for 2 specific goals. First as a webmail backend, we have 10 servers for that. We have a second group of servers for normal imap access.
Servers: 20 dual 3.x ghz FreeBSD 4.10 RAM: 4GB each Number of Users: 40.000+ daily users, 150.000 regular users, 500.000+ uses it once in a while Authentication: pam/radius Mailboxes sizes: customers get a 500MB mail quota, we use multi terrabyte netapp fileservers. Dovecot version: 1.0 beta4 I think, we cant keep up :) Email client software: anything you can imagine, and then some things you cant imagine.
We arent having a lot of problems. If we did, we'd be flooded with helpdesk calls. Our main problem right now is that we use 2 namespaces (one is historical) and one of those 2 uses mbox. Combined with filesystem quotas that are necessary for us this produces some problems when the quota is reached. Basically dovecot stops working at that point.
We're in the process of dropping support for that mbox namespace but with so many customers thats not easy. So until that time we have to help customers fix their quota on that FS if they fill it up.
There also seems to be the occasional index corruption causing faulty information to come from dovecot. For that reason we dont use indexes on our webmail cluster unfortunately (well, we use memory indexes). The IMAP service isnt fully in production yet so we can play around more there, and occasionally customers still experience what are results of index failures. Maybe beta7 will fix it :)
Cor