Short-life emails in iOS
My wife and I each have an iPhone, plus we share an iPad and iMac between us. Why is it that our iPhones and iPad only retain emails for seven days on one account, but the iMac keeps them for much longer?
This reflects the different ways in which your iMac and iOS devices are set up to access that email account. When you connect an email account to Mail on either platform, it should check with your mail server whether it’s able to use the IMAP protocol; these days, this is preferable to POP, as it makes email easier to manage when an account is accessed from multiple devices.
Your iOS devices are set to collect new messages using the IMAP protocol, which leaves them on your mail provider’s server, at least until you manually delete them.
For some reason, Mail on your iMac has been configured to collect incoming messages using POP, which downloads the messages and stores them locally. Usually, a client using POP will remove messages from the server after downloading them. Mail has an advanced POP setting that enables you to specify when messages should be removed from the server, though. You can set it to never happen, or to occur after a specific duration, such as a week, to give other devices time to download items.
However, we recommend setting up email accounts to use IMAP. If your provider limits your online storage, move old messages to a local mailbox in Mail on your Mac (Mailbox > New Mailbox, and set location to On My Mac). Unless explicitly excluded, this mailbox will be included in your Time Machine backups. The downside is that if you need to refer back to those messages, you must do so on your Mac. Mail has an advanced setting where you can specify how long it leaves messages on the server after they have been downloaded.