Known Protocol Limitations
**************************

This chapter lists known limitations to protocols commonly in use
today, that may impact your deployment.


POP3 and Mailbox Locking
========================

POP3, as described in **RFC 1939**, requires a mailbox to be locked by
a POP3 session.

As such, when POP3 is used simultaneously across multiple clients, and
a common set of mailboxes, an error similar to the following would
occur:

   Mailbox locked by POP server.

The exact error message may be subject to the specific error message a
client application wishes to display.


Cyrus IMAP POP3 Implementation
==============================

The Cyrus IMAP POP3 server implementation does not have the
aforementioned problem of POP3 sessions locking mailboxes. As of
version 2.4.0, Cyrus IMAP allows multiple POP3 sessions to operate on
a single mailbox by providing a *virtual* snapshot of the mailbox, and
all operations are executed to this snapshot. A safety mechanism
ensures no messages are deleted until after all existing operations
have closed the mailbox - including IMAP, LMTP and POP.