dovecot - Sep 2015 - apple mail fails to connect intermittently

Hi

(New to dovecot)

I am experiencing intermittent connection failures with both apple
mail and ios mail. No specific errors and I cannot find relevant
information on the mac logs or on my server.

It spontaneously corrects.
restarting dovecot - no dice

It just happened now (again), this time there might have been a
reason, I just dovecot-lda'd some large mailboxes from mbox to
Maildir. But as I said, it spontaneously corrects, so is
downloading now.

While this connection error is occurring, then I can connect to
dovecot via a remote mutt session, via web imap client and via
outlook.

Is this a known issue?

FWIW:
 % doveconf -n
# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 3.19.0-26-generic i686 Ubuntu 15.04
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes

namespace inbox {
  inbox = yes
  location   mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  mailbox virtual/All {
    special_use = \All
  }
  prefix }
passdb {
  driver = pam
}
postmaster_address = postmaster at biovolt.nl
protocols = " imap lmtp"
ssl_cert = </etc/dovecot/dovecot.pem
ssl_key = </etc/dovecot/private/dovecot.pem
userdb {
  driver = passwd
}
protocol lda {
  info_log_path = /tmp/dovecot-deliver.log
  log_path = /tmp/dovecot-deliver-errors.log
}

Thanks
Eric