"Mailbox is locked by POP server"

[Michael Sims]

Jure Pecar wrote:
> Recently I have more and more users complaining about errors accesing
> their mailboxes via pop. I woundn't (couldn't) really care about
> them, but one of them is now my boss :)  
> 
> So I started looking into it and we came up with the following
> procedure to recreate the problem: 
> 
> open outlook
> let it connect to the mail account
> let it start download new msgs
> kill it via "end task"
> 
> Cyrus keeps the connection open and all following attempts to log in
> fail. 

Are you using any "transparent" network devices (say, an intrusion detection device)?  I had a problem similar to yours but mine was caused by an IDS.  Relevant thread:

http://marc.theaimsgroup.com/?l=info-cyrus&m=108750460517979

I would suggest trying to find the pop3d process that is stuck and strace it to see what it is doing...