POP3 and Leave a copy on the server: how it works and what goes wrong

E-mail·· 4 views· 4 min read

The short answer: POP3 was made to download and delete. The “Leave a copy of messages on the server” option makes the program download without deleting, and remember what it already fetched through an identifier per message (the UIDL). It works, but it has consequences: the mailbox on the server keeps filling up, a device without the option “steals” mail from the others, and if the program loses its memory of the UIDLs it downloads everything again as duplicates. If you read mail in more than one place, IMAP solves all of this at the root: POP or IMAP.

What goes wrong, and why

Symptom Why it happens
Hundreds of old e-mails arrive again, as duplicates The program lost its list of UIDLs already downloaded: a new profile, a reinstall, a damaged data file, or the mailbox was migrated and the new server gave the messages new identifiers.
The mailbox on the server is full, though I delete everything on the computer With “leave a copy” and no time limit, messages stay on the server for ever. Deleting on the computer does not delete them there, unless you turn that on separately. See your mailbox is full.
An e-mail shows up on the computer but never on the phone One of the devices does not have “leave a copy” and deletes from the server whatever it downloads. Whoever checks first gets the message; the others never see it.
What I send does not appear in Sent on the other device or in webmail POP3 only reads the Inbox. Sent items stay in the program that sent them.
A message in the spam folder in webmail never reaches the program POP3 does not see folders. Whatever a server filter put in the spam folder, or in another folder, stays there.
I deleted in webmail and it came back on the computer (or the other way round) That is normal with POP: each side has its own copy and they do not talk.

Setting up POP without surprises

1 First decide whether you really need POP. If you use a phone, a computer and webmail, switch to IMAP. The POP or IMAP guide explains how, without losing what you already downloaded.
2 If you stay on POP with several devices, turn on “leave a copy” on all of them but one. In classic Outlook it is under File > Account Settings > Change > More Settings > Advanced; in Thunderbird under Account Settings > Server Settings. The names change with the version and language.
3 Give the copy a time limit on the main device: “remove from server after” a number of days. The other devices have that long to download it; after that the server is cleared. Pick the limit with the device that connects least often in mind.
4 Turn on “remove from server when deleted from Deleted Items” (or its equivalent), if you want what you delete on the computer to leave the server too.
5 Check the spam folder in webmail now and then, because POP will never show you what is in it: accessing your webmail.
6 If you already have duplicates, do not delete the account to start over: with POP, the computer may hold the only copy. For Outlook, see duplicate e-mails in Outlook.
With POP, the copy on your computer may be the only one. If removing from the server is on and the computer breaks or is stolen, those messages are lost. Back up the program data file, or keep a copy as explained in keeping your own copy of your e-mail.
To see what is really on the server, open webmail, or count the messages by hand with the POP3 STAT command: testing IMAP, POP3 and SMTP by hand.

POP is downloading duplicates or the mailbox filled up and you do not know why? Tell us the program, the devices you use and the account address.

Open a support ticket

SEE ALSO

POP or IMAP: which to choose, and what you lose by picking wrong

Your mailbox is full: what to delete and how to stop it happening

Duplicate e-mails in Outlook: where they come from and how to stop them

Keeping your own copy of your e-mail: how to back up a mailbox

How to set up your e-mail in Thunderbird

Testing IMAP, POP3 and SMTP by hand with openssl s_client: a guide for technical users

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