[Dovecot] sporadic slow/non response, problems copying to, sent folder

[Danno Coppock]

>> symptoms:
>>   1. Coming back to the idle client, and clicking on a newly arrived 
>> message, often a long delay (seems like minutes, sometimes) will
>> occur before the message loads from the server.
>>   2. Sending messages, as often as not, the client will stall,
>> sometimes indefinitely, on "copying message to sent folder".
>>     
> Does this happen when using a client which is on the same LAN as
> the server?  This kind of behavior looks like a network issue.
>   
You called it.  We access the server over the Internet.  How did I forget to provide that small detail?  However, that's not the slowness I speak of.  Perhaps a better term is long delay, like waiting on some timeout, before message loadding (in symptom 1., above).

So are there parameters to tune, that will make dovecot behave nicer?  I do not believe this to be connectivity outages between our clients and the server.  For one, we never experienced these symptoms with UW-IMAP.

I just duplicated the stall on copying to Sent folder.  This is probably the more prevalent, and annoying of the two problems.

facts:
* There was only one active client.
* The client was T'bird, v1.5.0.10, running on XP.
* This particular test had a 3Mb attachment.
* The mail was delivered immediately to the single, remote recipient (after a ~20 seconds of 3Mb upload).
* The "Copying to Sent Folder" dialog stayed up for (no exaggeration) 15-minutes, and then completed; the message is in the Sent folder now.

suspicion, from months of experiencing the behavior:
* Attachments are usually involved.

How does the config look? Are the mbox lock settings appropriate? I was 
somewhat shooting in the dark there.

    * Sendmail 8.13.6/8.13.6
    * $sudo sendmail -bt -d0.10 < /dev/null | grep HASFLOCK
             OS Defines: BSD4_4_SOCKADDR HASFCHOWN HASFCHMOD HASFLOCK
 
    mbox_read_locks = flock
    mbox_write_locks = flock

What else can I provide?  netstats?  tcpdumps?  Are there logs/debugging appropriate to enable?
 
Thanks for any ideas, Danno