Ok, we updated to dovecot 2.0.9, and we're still seeing the "net_connect_unix(imap) failed" problem happen around peak login time, 9-10am.
dovecot -n # 2.0.9: /etc/dovecot/dovecot.conf # OS: Linux 2.6.32-71.14.1.el6.x86_64 x86_64 Red Hat Enterprise Linux Server release 6.0 (Santiago) auth_master_user_separator = * auth_mechanisms = plain login default_client_limit = 5000 default_process_limit = 5000 disable_plaintext_auth = no mail_fsync = always mail_gid = 501 mail_location = maildir:~/Maildir:INDEX=/var/indexes/%n mail_nfs_storage = yes mail_plugins = quota mail_uid = 501 passdb { args = /etc/dovecot/passwd.masterusers driver = passwd-file master = yes pass = yes } passdb { args = /etc/dovecot/dovecot-ldap.conf driver = ldap } plugin { quota = maildir:User quota quota_rule = *:storage=200M } service anvil { client_limit = 15000 } service auth { client_limit = 32768 unix_listener /var/spool/postfix/private/auth { group = postfix mode = 0660 user = postfix } user = root } service imap-login { process_min_avail = 12 service_count = 0 } service pop3-login { service_count = 0 } shutdown_clients = no ssl_cert = </etc/pki/postfix/certs/nexus.pem ssl_key = </etc/pki/postfix/private/nexus_key.pem userdb { args = /etc/dovecot/dovecot-ldap.conf driver = ldap } protocol imap { mail_plugins = quota imap_quota }