How your mail syncs

The app keeps a copy of your mail on this device so the list opens instantly and search works without waiting for the network. Your provider still holds the original. Nothing here deletes mail from the server.

Choose how far back to download

Each account decides for itself how much of its history to keep locally. Open Settings, choose Accounts, then the account, and look for Sync depth.

Under Download you can pick:

• Last 3 months
• Last 6 months
• Last 9 months
• Last 12 months
• Last 24 months
• All time

This setting decides what is on the device. Mail older than the window is not downloaded, and opening the folder it is in does not fetch it either, so it does not appear in the list and local search does not find it. Your provider still has all of it, and widening the window brings it down.

Each account is separate, so a work account can keep two years while a mailing-list account keeps three months.

What gets downloaded, and when

Syncing happens in two parts, and knowing which is which explains most of what you see.

The list arrives first. For every message in your chosen window the app fetches the small things a list row needs: who it is from, the subject, the date, whether it is read or flagged, whether it has an attachment, and a short preview. This is quick even for a large mailbox, which is why your inbox fills in before anything else finishes.

The message itself follows. The body text, the formatting and the attachments are larger, so they are fetched separately. The app works through the messages in your window in the background, so by the time you open one it is usually already there. If it is not, opening the message fetches it right then.

Two consequences worth knowing:

• A message can appear in the list a moment before its body is ready. That is the second half catching up, not an error.
• The background pass fetches each message whole, attachments included, so that the whole window can be read and searched without a connection. A mailbox full of large attachments therefore does use disk, and how far back you sync is what bounds it.

Marking a message read or flagging it changes only the small part, so neither re-downloads anything: the message itself never changes once your provider has it. Moving a message can cost more, because some providers treat a message in a new folder as a new message.

Start the local copy over

If the local copy ever looks wrong, in a way that a normal sync does not fix, you can throw it away and download everything again.

1. Open Settings and choose Advanced.
2. Choose the reset option. Its description reads: "Clear the locally cached mail and download everything again from the server. Queued sends are kept."
3. Confirm.

Messages you have written that are still waiting to send are kept, so a reset cannot lose a queued message.

This is worth trying if the list shows something that is clearly out of date and stays that way after the app has synced. If it does not help, the next page is the one you want.