POP3

POP3 on 995 (implicit TLS). Full email address as the username, mailbox password as the password.

Port 110 listens but cannot be used: there is no server-side STLS, and Corsair will not take a credential unencrypted. See TLS.

It exists for the clients that still want it.

Commands#

That is the whole of RFC 1939 plus UIDL and TOP. STLS is present in the command table but cannot complete; use 995.

INBOX only#

POP3 has no concept of folders. A session sees INBOX and nothing else.

Mail filed elsewhere by a Sieve filter — into Newsletters, into Junk — is invisible over POP3. That surprises people who set up a filter and then wonder where their mail went.

Deletions commit at QUIT#

DELE marks. QUIT commits. A connection that drops before QUIT deletes nothing, which is the protocol working as intended.

RSET unmarks everything in the current session.

UIDL#

UIDL gives each message a persistent identifier, which is how a client implements "leave messages on the server" — it records the identifiers it has already downloaded.

Every client worth using supports it. A client that does not has to delete as it goes, or download everything every time.

When POP3 is right#

A device that should drain a mailbox to local storage. An archival machine, a system that ingests mail into something else, a printer.

A genuinely single-device setup where server-side state is not wanted.

When it is wrong, which is usually#

POP3 downloads and, by default, deletes. It has:

• No folders
• No flags shared between devices — read on your laptop is unread on your phone
• No server-side search
• No concept of a second client

Two devices both using POP3 against one mailbox race each other for the mail. Whichever polls first gets it.

Use IMAP unless you have a specific reason not to. If you must use POP3, turn off "delete from server" in the client unless removal is the point.

Testing by hand#

openssl s_client -connect mail.example.com:995 -crlf

USER you@example.com
PASS your-password
STAT
LIST
UIDL
TOP 1 10
RETR 1
QUIT

Replies are +OK or -ERR, which makes POP3 the easiest of the mail protocols to debug by hand.

Same store#

POP3 reads the same rows as IMAP, JMAP, and the webmail. A message deleted over POP3 is gone from all of them — it is the same message, not a copy.

That is worth saying explicitly, because the POP3 mental model of "downloading my mail" suggests otherwise. There is one copy, and DELE plus QUIT deletes it.