Open the app, then tap the menu button (three horizontal lines, top left), and choose Settings. Tap Add account, then work through the setup process on screen: You’ll be asked to choose an email provider, like Outlook or Yahoo (tap Other if yours isn’t shown), and then you’ll need to enter your login credentials.
On the final screen of the account setup, you can choose how often the account syncs to the Gmail app, and whether new emails prompt notifications. When you’re done, you can start sending and receiving emails from the Gmail app.
Tap the app menu button and choose All inboxes to see messages from all your accounts mixed in with your default Gmail account—the search box at the top will search through all your accounts, the sent folder will show outgoing emails from all your accounts, and so on. If you need to see or search through an individual account, tap your Google account avatar (top right), then choose the account from the list.
Open up the main Settings app for iOS, then choose Mail. Tap Accounts, then Add Account to see a list of options including Microsoft Outlook and Google. If you don’t see the one you want, tap on the Other button.
Follow the steps shown on screen to set up another email account on iOS. You’ll need to enter the username and password, then you’ll be asked if you want to sync contacts, calendars, and other data from the account.
Once you’ve added the new account, it will appear on the Accounts screen, where you can use the Fetch New Data menu to choose when to sync new emails. From the Mailboxes screen in the Mail app itself, you can tap All Inboxes to see all your messages rolled together in chronological order, or tap a specific account name to manage messages from that account alone.
Open up the Mail app from the Start menu (use the search box if you can’t immediately find it), click the cog icon (bottom left of the sidebar), and look for a Manage accounts option on the right. Click it, and then select the Manage accounts option, followed by Add account. You can then pick your account type if you see it, or hit Other account if you don’t.
Work through the subsequent dialog boxes to enter your login details for the new account. In most cases it should only take a couple of minutes—you can then select it from the Manage accounts screen to rename it or change the email sync settings.
While you’re using Mail with multiple email accounts, all the account names will be listed on the left side of the program, so click any name to view email from that account. You can’t view all your emails from multiple accounts together in one continuous stream through—you’ll need a different client for that (see below).
Open up Mail then click the Mail menu and Add Account. These are the accounts (like Google and Yahoo) set up to work with various aspects of macOS, including the email app. If you want to associate a new email address, click the relevant entry on the list (use Add Other Account if you can’t see the client you use).
The setup progress is straightforward enough: enter the username and password you usually use to sign into the email account, and choose which types of data to sync. With Google and Microsoft accounts, for instance, you can sync contacts and calendar information as well as messages.
Source : https://www.popsci.com/how-to-sync-emails-gmail-android-outlook-ios/