# 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.