If your Mac is connected to the Internet but suddenly stops sending email, you should generally try again later, and if necessary work with your email provider to resolve. If you're using Apple's iCloud Mail, learn how to resolve issues with iCloud Mail.
Check for status messages
You might see a status icon such as a lightning bolt or warning symbol next to your account's inbox in the Mail sidebar, or in the upper-right corner of the Mail window. Click it to learn more.
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- If the status is Network Offline, make sure that your Mac is connected to the Internet, then try again.
- If the status is Login Failed, check with your email provider to be sure that your account is set up correctly on your Mac.
- If Mail asks for your password, but entering the password doesn't help, find out why your email provider is rejecting your password.
- If your email service is temporarily unavailable, your email provider might have a status message on their website. For example, Apple's system status page shows the status of iCloud Mail. Status pages are also available for other email services, such as Gmail and Outlook. Some brief service outages might not appear on a system status page, so you might want to wait a few hours and try again.
Test using webmail
Question: Ever since installing macOS 10.14 Mojave I have only gotten my primary Gmail account up and running. My secondary Gmail account that I only want to use for eMail though macOS Mail.app still isn't setup properly: I keep having a 'Login Failed' warning displayed in the upper right corner of my All Mailboxes window. This automatically adds the account to the default iOS mail app, but it doesn't work for MFA users because it is not configured with OAuth/modern authentication, and this causes all kinds of problems for the users. I believe the full version of Intune MDM has an option to enable OAuth now, but it hasn't been addressed in Office 365 MDM for.
Many email providers offer webmail, which is a way to use email with a web browser such as Safari, instead of with an email app such as Mail. For example, if you're using an iCloud Mail account, you can use iCloud.com to send and receive email.
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Webmail is a good way to verify that your account is valid and you can sign in to it from the web, but it doesn't mean that your account is set up correctly in Mail. Your email provider can help you verify your email settings and check for other issues that affect sending or receiving email.