Apple mail doesn't download messages after upgrade to ios 13.3.1

Google issue. As admin to a Google domain, I received the following last year. Even though the date says 2021, I'm thinking they had a typo; already had to reconfigure all my G-Suite accounts to OAuth authentication as described:

Starting February 15, 2021, G Suite accounts will only allow access to apps using OAuth. Password-based access will no longer be supported.



Dear Administrator,

We’re constantly working to improve the security of your organization’s Google accounts. As part of this effort, and in consideration of the current threat landscape, we’ll be turning off access to less secure apps (LSA) — non-Google apps that can access your Google account with only a username and password, without requiring any additional verification steps. Access through only a username and password makes your account more vulnerable to hijacking attempts. Moving forward, only apps that support a more modern and secure access method called OAuthwill be able to access your G Suite account.

Access to LSAs will be turned off in two stages:

June 15, 2020- Users who try to connect to an LSA for the first time will no longer be able to do so. This includes third-party apps that allow password-only access to Google calendars, contacts, and email via protocols such as CalDAV, CardDAV and IMAP. Users who have connected to LSAs prior to this date will be able to continue using them until usage of all LSAs is turned off.

February 15, 2021- Access to LSAs will be turned off for all G Suite accounts.

What do I need to do?

<<Admin instructions suppressed>>

User instructions

If you are using an app that accesses your Google account with only a username and password, take one of the following actions to switch to a more secure method and continue to access your email, calendar, or contacts. If you do not take one of the following actions, when LSA access is discontinued after February 15, 2021, you will begin receiving an error message that your username-password combination is incorrect.

Email

If you are using stand-alone Outlook 2016 or earlier, move to Office 365 (a web-based version of Outlook) or Outlook 2019, both of which support OAuth access. Alternatively you can use G Suite Sync for Microsoft Outlook.

If you are using Thunderbird or another email client, re-add your Google Account and configure it to use IMAP with OAuth.

If you are using the mail app on iOS or MacOS, or Outlook for Mac, and use only a password to login, you’ll need to remove and re-add your account. When you add it back, select “sign in with Google” to automatically use OAuth.

...

Thanks for choosing G Suite.

—The G Suite Team

Try contacting google or look at their support site.