UIDL

UIDL, or Unique Identifier Listing, is a command used in the Post Office Protocol version 3 (POP3) to identify individual messages in a mailbox. It provides a per-message identifier that remains stable for the lifetime of the message on the server, allowing clients to determine which messages they have already downloaded.

How it works

- UIDL can be issued with no argument to request a list of all messages in the mailbox, along with each message’s unique ID (UID).

- UIDL n can be used to request the UID for a specific message by its number n.

- The server responds with a status line followed by one or more lines containing the message number and its UID, ending with a single dot on a line by itself.

Usage and behavior

- UIDs are intended to be unique within a single mailbox and persist across sessions, enabling clients to avoid re-downloading messages.

- UIDs are not guaranteed to be globally unique across servers, and they may be reused in some server implementations if messages are deleted and new messages are added.

- POP3 servers that do not implement UIDL behave like older servers that require clients to rely solely on message sequence numbers, which can lead to re-downloading messages in subsequent sessions.

Relation to other protocols

- UIDL is specific to POP3; it is complementary to the use of message headers (such as Message-ID) but is not the same as IMAP’s UID system (which operates within the IMAP protocol and has its own extensions).

- Many modern clients prefer IMAP for robust message synchronization, but POP3 with UIDL remains supported by some servers and clients for backward compatibility.

Limitations

- Not all POP3 servers implement UIDL; clients should be prepared to fall back to sequence-number tracking when UIDL is unavailable.

- Since UIDs may be reused after deletion, they should not be treated as permanent identifiers beyond the mailbox lifetime.