[Dovecot] Proxy connection timeouts

[Kelsey Cummings]

We are seeing a few (0-15) proxy failures like the following out of ~3m 
successful proxied connections a day. Average session creation load over 
our peak hour is about 47/sec.  The backend servers aren't logging 
anything that would suggest any internal problem like insufficient 
processes to handle the load.  It doesn't seem to happen when 
utilization is lowest at night.

dovecot: imap-login: Error: proxy(foo): connect(1.1.1.1, 143) failed: 
Connection timed out (after 63 secs)

I'm curious if anyone else has seen any similar problems or has any 
suggestions.

# dovecot -n
# 2.1.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-279.5.2.el6.x86_64 x86_64 Scientific Linux release 
6.3 (Carbon)
auth_master_user_separator = *
auth_username_format = %Ln
auth_verbose = yes
auth_verbose_passwords = sha1
auth_worker_max_count = 64
mail_fsync = always
mail_log_prefix = "%s(%u): session=%{session} "
mail_plugins = stats zlib
maildir_very_dirty_syncs = yes
mmap_disable = yes
passdb {
   args = /etc/dovecot/master-users
   driver = passwd-file
   master = yes
}
passdb {
   args = imap
   driver = pam
}
plugin {
   lazy_expunge = DELETED_MESSAGES.
   stats_refresh = 30 secs
   stats_track_cmds = yes
}
protocols = imap pop3
service anvil {
   client_limit = 10000
}
service auth {
   client_limit = 10000
   vsz_limit = 512 M
}
service doveadm {
   inet_listener {
     port = 1842
   }
   unix_listener doveadm-server {
     mode = 0666
   }
}
service imap-login {
   inet_listener imap {
     port = 143
   }
   inet_listener imaps {
     port = 993
     ssl = yes
   }
   process_limit = 7000
   process_min_avail = 32
}
service imap-postlogin {
   executable = script-login -d /etc/dovecot/bin/sonic-imap-postlogin
   user = $default_internal_user
}
service imap {
   executable = imap imap-postlogin
   process_limit = 4096
   vsz_limit = 512 M
}
service pop3-login {
   inet_listener pop3 {
     port = 110
   }
   inet_listener pop3s {
     port = 995
     ssl = yes
   }
   process_limit = 2000
   process_min_avail = 32
}
service pop3-postlogin {
   executable = script-login -d /etc/dovecot/bin/sonic-pop3-postlogin
   user = $default_internal_user
}
service pop3 {
   executable = pop3 pop3-postlogin
   process_limit = 4096
}
service stats {
   fifo_listener stats-mail {
     mode = 0666
   }
}
shutdown_clients = no
ssl = required
ssl_ca = </etc/dovecot/ssl/gd_bundle.crt
ssl_cert = </etc/dovecot/ssl/imap.sonic.net.crt
ssl_key = </etc/dovecot/ssl/imap.sonic.net.key
ssl_parameters_regenerate = 1 days
syslog_facility = local0
userdb {
   driver = passwd
}
verbose_proctitle = yes
protocol imap {
   imap_id_send = support-url support-email
   mail_max_userip_connections = 20
   mail_plugins = stats zlib mwi_update mail_log notify imap_stats imap_zlib
   ssl_ca = </etc/dovecot/ssl/gd_bundle.crt
   ssl_cert = </etc/dovecot/ssl/imap.sonic.net.crt
   ssl_key = </etc/dovecot/ssl/imap.sonic.net.key
}
protocol pop3 {
   mail_plugins = stats zlib lazy_expunge
   pop3_fast_size_lookups = yes
   pop3_uidl_format = %f
   ssl_ca = </etc/dovecot/ssl/pop.sonic.net.ca-bundle
   ssl_cert = </etc/dovecot/ssl/pop.sonic.net.crt
   ssl_key = </etc/dovecot/ssl/pop.sonic.net.key
}