How does Gmail POP work?

POP, short for the Post Office Protocol, is used to sync email from Gmail to any compatible mail client, such as Outlook, Thunderbird, or Apple Mail. Like IMAP, POP is not a Google product; it's a standardized, RFC-compliant protocol that any email service or client can choose to be compatible with.

Gmail users can use either POP normal mode or recent mode to sync their mail. If you're syncing your mail to one mail client, you should use normal mode. If you're syncing mail to a few different mail clients, then you should use recent mode.

How does normal mode work?

A POP client session starts with your mail client (Thunderbird, Outlook, Sparrow, etc.) asking your Gmail mailbox for a list of messages that haven't yet been downloaded. After Gmail provides the list of messages to your mail client, your client will begin downloading them. In POP normal mode, Gmail provides a list of about 250 of the oldest messages that have not yet been downloaded (Spam and Trash are excluded). Once a message is downloaded, Gmail marks it as 'popped'.

Messages sent from an email client don’t appear in the POP mail client in normal mode, but they do appear in recent mode. Messages sent from email clients are marked as popped, so they aren’t downloaded into the Inbox of POP mail clients.

Note: Messages sent from the web UI appear in the POP mail client in both normal mode and recent mode.

Why "about 250" messages? When your POP client asks Gmail for the list of messages, Gmail retrieves the oldest 250 conversations that haven't been popped yet. Because a conversation in Gmail can consist of more than one message, your mail client will probably download more than 250 individual messages.

What happens to my messages in Gmail after they've been popped?

That depends on which behavior you've selected in the When messages are accessed with POP setting in Gmail. You can choose to archive, delete, or keep you keep your messages in your inbox.

1. Sign in to your Gmail account.
2. Click the  gear in the top right.
3. Select Settings.
4. Click Forwarding and POP/IMAP.
5. Select the drop-down menu next to When messages are accessed with POP and choose which setting you prefer.
6. Click Save changes.

Regardless of which behavior you select, any downloaded message will be marked internally as 'popped' and will not be downloaded again. If however, your mail client crashes unexpectedly, the message will be re-downloaded.

What happens if my email client crashed while downloading messages?

Gmail considers messages as downloaded once they've been downloaded using the RETR command (as explained in the protocol documentation). This means that once a message is downloaded by the POP client, if the POP session ends normally (with the QUIT command), Gmail will mark the message as popped (and it will no longer appear when POP clients get the list of message) - even if the client didn't explicitly specify the DELE command after downloading the message. If the session didn't end normally (e.g. the connection dropped and no QUIT command was received), the messages downloaded during the session won't be marked as popped.

After your client downloads the messages, it asks Gmail for a list of messages again. Gmail will again provide a list of the next oldest 250 conversations that haven't been popped yet. Eventually, your mail client will download all of your messages in Gmail, although this process may take a while, depending on the size of your Gmail mailbox.

How does recent mode work?

A POP client session starts with your mail client (Thunderbird, Outlook, Sparrow, etc.) asking your Gmail mailbox for a list of messages that haven't yet been downloaded. After Gmail provides messages to your mail client, your client begins downloading them. In POP recent mode, Gmail will present all messages from All Mail (Spam and Trash are excluded) from the last 30 days. This means that multiple POP clients can download the same messages, and they will all see all messages (as long as they check the mailbox at least every 30 days).

What happens to my messages in Gmail after they've been popped?

When using recent mode, 'popped' messages (downloaded in normal mode) will still be shown to mail clients. This means that even if one POP client (that uses normal mode) marks a message as popped, another POP client (that uses recent mode) will still be able to see the message (unless you've set Gmail to delete messages that have been downloaded via POP in the When messages are accessed with POP option, in which case the message will be sent to Trash after it's been downloaded by the POP client in normal mode).

Unlike in normal mode, you must set your POP client to leave messages on the server (and not delete them), because when a POP client issues a DELE (delete) command in recent mode, it is sent to Trash in Gmail, regardless of the user's When messages are accessed with POP setting. If one of the POP clients deletes messages, they won't be visible to the other POP clients ever again (unless moved out of Trash).

Re-downloading messages and excluding existing messages

After a message has been moved to your mail client from Gmail, you can't re-download it to any mail client in the future. For example, if you have to remove and reinstall your mail client from scratch, you can't download previously 'popped' messages unless you select the Enable POP for all mail (even mail that's already been downloaded) option in Gmail settings.

1. Sign in to your Gmail account.
2. Click the  gear in the top right.
3. Select Settings.
4. Click Forwarding and POP/IMAP.
5. Click Enable POP for all mail (even mail that's already been downloaded).
6. Click Save changes.

After enabling this setting, Gmail clears the 'popped' status from all messages. Your mail client will then see the oldest 250 messages in your mailbox, assuming you're using POP normal mode, and can download them again as explained above.

If you select the Enable POP for mail that arrives from now on setting, Gmail will simply mark all messages currently in your mailbox as 'popped'. As a result, your mail client will only download messages that you receive after choosing this option.