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The short answer: with IMAP the mail lives on the server, and what your program shows is a reflection of three things: the folders (which exist on the server), each message’s flags (read, answered, flagged, deleted) and notifications. “Instant” (push) e-mail comes from a command called IDLE: the program keeps a connection open and the server tells it when something arrives. But IDLE usually watches one folder at a time, which is why the Inbox is instant and other folders only update every so often.
The parts of IMAP and what they do
| Part | What it is | What goes wrong |
| Folders | They exist on the server. The program shows only the subscribed ones, and recognises the special ones (Sent, Trash, Junk, Drafts) by attributes the server announces. | A folder created in webmail does not appear on the phone (it is not subscribed), or the program makes its own “Sent Items” next to the server “Sent”. |
| Flags | Labels per message: \Seen (read), \Answered, \Flagged, \Draft and \Deleted (marked for deletion). They live on the server, and every device sees them. | Reading on the phone marks it read on the computer: that is the right behaviour. If it does not happen, one of the accounts is POP. |
| \Deleted and EXPUNGE | Deleting in IMAP takes two steps: flag it \Deleted, then expunge, which removes it for good. Many programs move to Trash instead. | Struck-through messages that never go away and keep using space: the program flagged them but did not expunge. |
| IDLE (push) | The program says IDLE and listens on one folder; the server sends a line when something arrives or changes. | Other folders are only seen when the program checks them. On a phone, the system may cut the connection to save battery, and push turns into a periodic check. |
| Simultaneous connections | Each watched folder and each device opens its own connection to the server. | With many devices and watched folders at once, the server may refuse new connections (“too many connections”). |
Getting folders and push to behave
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| Deleting in IMAP deletes everywhere. Because the message exists only once, on the server, deleting it on the phone and expunging removes it from the computer and from webmail too. If you want a copy of your own, make one on purpose: keeping your own copy of your e-mail. |
| Many devices, many connections. If you see connection errors only when everything is open, close the program on a device you no longer use, or watch fewer folders. |
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A folder does not update on one device but is right in webmail? Tell us the program, the device and the folder name. Open a support ticket |
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