Mailbox is locked by POP server

[Zhang Weiwu]

I am sorry for re-posting what Igor Belikov posted on this list two
years ago (and unanswered) because I got the same problem and the answer
isn't really obvious (googling around can lead me some really complex
discussion on how to detect and shut down the expired/dead connection
but not direct answer)

here is the question: 

Hello info-cyrus,

  We have clients that get his (or her) mail via dial-up connections.

  If dial-up session terminates abnormally (while getting mail) and
  client redial and try to continue get his mail - he receive error:

  Mailbox is locked by POP server

  I understand what is the problem: Cyrus don't see ending previous
  POP session and think that client still using mailbox, and don't
  allow him begin new session.

  Is it possible to turn this behavior off? We have enough dial-up
  clients, and nobody of them using IMAP - only POP3.

I know that POP3 do not allow concurrent connection, as written clearly
in RFC. Problem being our customer use really damn stupid email client
that looks occasionally dropping connection without notifying server,
and other email service providers here in China seems to handle this
particular email client pretty well. The email client also doesn't
support any of TLS, IMAP or LDAP and is very popular here (popular for
non-technical reason of course) that if we don't support it, customers
prefer to choose another email service provider who can. "I am used to
use xxx product" is such a strong reasoning from customer that service
providers cannot simply ignore and suggest Thunderbird & IMAP (I already
suggested many times).

Best regards
Zhang Weiwu