[Dovecot] Dovecot Hangs while mutile users download mail for same account using pop3

[Robert JR]

Hello,

I have a weird problem in dovecot, Dovecot Hangs while multiple users 
download mail for same account using pop3

Three persons use 1 same email , and three of them use outlook express 
to check
That specific mail .. Also some times one of the three users check the 
mail for this
Account using imap (squirrel mail)

And here comes the problem. suddenly all the three users are unable to 
check the mail And when the outlook express connect to the pop3 server, 
they are unable to disconnect And the outlook express keep asking "the 
server is not responding for 60 mins would you like to wait ?

i checked /var/log/maillog, i can see the following errors

dovecot: pop3(sales): Disconnected: Storage error during logout.

to=<sales at domain.com>, orig_to=<sales at domain.,com>, relay=local, 
delay=357, delays=338/0.01/0/19, dsn=4.2.0, status=deferred (cannot 
update mailbox /var/mail/sales for user sales. unable to lock for 
exclusive access: Resource temporarily unavailable)

When i used lsof | grep sales
pop3      4278   sales   10u      REG                8,3 22897673   
12615705 /var/spool/mail/sales


i went to /home/sales/mail/.imap/INBOX , but i didnot find any lock 
files

-rw------- 1 sales sales   944 Oct  7 13:35 dovecot.index
-rw------- 1 sales sales 49152 Oct  7 16:57 dovecot.index.cache
-rw------- 1 sales sales 14044 Oct  7 16:57 dovecot.index.log


1- So what is exactly the problem? is the problem that three users are 
checking the same exact mail ?
2- is the problem that 2 users checking mail using pop3 and the third 
one checking it using imap insame time ?
3- is dovecot locking the inbox file while one of the users getting the 
mail ?
4- What is the cause of the problem ?

Please help
Regards

Robert JR