G: [Non-text content: thoughts; content_type=thoughts]

G: [Non-text content: reasoning_recap; content_type=reasoning_recap]

G: Yes. I checked specifically for something happening today (September 14), and I’m not finding evidence of a current widespread Gmail/Google IMAP outage that would explain this. So I would not start by changing random server settings.

The photograph actually gives us a very useful clue. The important part isn't “imap.gmail.com is not responding”; it's the lower line:

AUTHENTICATIONFAILED — Invalid credentials

That means the phone is reaching Gmail, but Gmail is rejecting the authentication that Apple Mail is presenting. Google documents “Invalid credentials” as an authentication/sign-in problem. citeturn0search1

What I would do, in this order

1. First test his Google account outside Apple Mail.
   On the phone, open Safari and go to Gmail and see whether he can sign in normally. Don't change anything yet. If Google asks him to verify himself, accept a security prompt, or enter a new password, do that.

2. Ask whether his Google password changed recently — even if he didn't deliberately change anything on the iPhone. This exact situation can occur after a Google password reset because Google can revoke the authorization token that Apple Mail was using. Google explicitly says that after a password change, the native iOS Mail app may stop syncing until the user authenticates again. citeturn0search9

3. If Gmail works in Safari but Apple Mail still gives this message, reauthorize the account. On a current iPhone, go to Settings → Apps → Mail → Mail Accounts and select the Gmail account. If it offers a sign-in/re-enter-password option, use that.

4. If it doesn't offer a useful reauthentication option, remove the Gmail account from the iPhone and add it back. Go to Settings → Apps → Mail → Mail Accounts → [his Gmail] → Delete Account. This only removes the account from the iPhone; it doesn't delete his Gmail account or mail from Google's servers. Apple confirms that explicitly. citeturn0search6

   Then use Add Account → Google — specifically choose Google, rather than Other or manually entering imap.gmail.com. Google itself recommends Settings → Mail → Accounts → Add Account → Google for an iPhone. citeturn0search3

There's another reason I particularly like that last solution here. Google has increasingly moved Gmail authentication away from having third-party mail clients simply store a Google username/password and toward Google's OAuth “Sign in with Google” process. citeturn0search10 If your dad's Gmail account was originally configured on this phone a long time ago or manually as IMAP, removing it and adding it again explicitly as Google gives Apple Mail a fresh Google authorization rather than trying to repair the old credentials.

So I wouldn't blame your dad yet. 😄 This error can spontaneously appear from his perspective even if he touched nothing in Mail: an expired/revoked authorization, Google security action, or password/account change elsewhere is sufficient.

If you've got his phone in front of you, I'd start with the Safari Gmail login before deleting anything. Tell me what happens there, and I can walk you through the shortest branch from that result rather than having you shotgun-troubleshoot the phone.

G: [Non-text content: thoughts; content_type=thoughts]

G: [Non-text content: reasoning_recap; content_type=reasoning_recap]

G: Yes — but simply opening or signing into the Gmail app should not normally break Apple Mail's authorization. Google treats the Gmail app and Apple's Mail app as separate clients; signing into one doesn't ordinarily revoke the other's access. citeturn0search0turn0search2

And your new information narrows this down considerably:

• His Google account/password works on the computer.
• It works through Google/Gmail on the iPhone.
• Apple Mail specifically is reporting AUTHENTICATIONFAILED.
• This settings screen looks like an older/manual-style IMAP configuration rather than giving you an obvious Google reauthorization button.

So yes, I would use “Delete Account” at the bottom of the screen you're showing me. Apple confirms that this removes the account from the iPhone; it does not delete the Gmail account or the mail stored at Google. citeturn0search3

Then I'd rebuild it cleanly:

Settings → Apps → Mail → Mail Accounts → Add Account → Google

The important part is choose “Google.” Do not choose “Other” and don't manually enter imap.gmail.com. Apple supports Google's automatic setup, and Google's own instructions specifically say to select Google when configuring Gmail in Apple Mail. citeturn0search7turn0search13

It should then throw him into Google's own sign-in/authorization screen. He signs in there, approves access, and Apple gets a fresh OAuth credential.

One interesting thing I found while checking: there have indeed been reports of Apple Mail losing its Gmail OAuth authorization while Gmail itself continues working, producing essentially the same imap.gmail.com error and being fixed by removing/re-adding the account. That's anecdotal rather than an official diagnosis, so I wouldn't conclude that's definitely what happened here. citeturn0search10

But given everything you've now tested, a broken/stale Apple-Mail-specific authorization is much more likely than a broken Gmail account. I wouldn't bother messing with SMTP or Advanced on the screen you're showing. Delete this copy from Apple Mail and add it back through Google.

G: [Non-text content: thoughts; content_type=thoughts]

G: [Non-text content: reasoning_recap; content_type=reasoning_recap]

G: Yes. I went looking specifically for the other plausible explanations before you hit Delete, including user reports, and there are several—but none makes deleting/re-adding look dangerous, and most actually converge on the same repair.

Given that Gmail works in Safari and on his computer, the alternatives I would keep alive are:

1. His Google password changed or Google performed a security reset. This is the strongest alternative. Google explicitly says a password change can revoke OAuth tokens belonging to third-party mail clients such as Apple Mail. Gmail in Safari can consequently work perfectly while Apple Mail suddenly stops. citeturn0search9

2. Apple Mail has an old/expired/corrupted Google authorization token without any password change. This fits your evidence extremely well. The actual server response in your photo is AUTHENTICATIONFAILED / Invalid credentials, meaning this isn't merely Apple failing to contact imap.gmail.com.

3. The account was originally configured using an older/manual IMAP authentication method. Your second photograph makes me somewhat more suspicious of this. Google has progressively eliminated older username/password authentication in favor of OAuth. Current Google guidance is to use an OAuth-capable client, and Apple says that if the provider's authentication method changes, removing and re-adding the account may be necessary. citeturn0search10turn0search0

4. An iOS/Apple Mail software problem. Google itself lists an outdated app or operating system as a possible reason a correct Google password gets rejected by a third-party application. citeturn0search3 So I'd check Settings → General → Software Update before deleting—not necessarily install an update immediately, just see whether he's substantially behind.

5. A temporary Google/Apple interoperability glitch. There are user reports of Gmail continuing to work on the web while Apple Mail's Gmail connection fails. That's possible, although what you're seeing is a pretty explicit authentication rejection rather than a generic network failure. citeturn0reddit36

6. Wrong IMAP username/server/SSL settings. Technically possible, but quite unlikely if nobody edited this account and it worked until today. I specifically would not start changing those settings, because that can turn one problem into several.

The thing I was particularly interested in checking was your “he opened the Gmail app and somehow kicked Apple Mail off” hypothesis. I can't find a normal Google mechanism by which simply signing into Gmail's own iOS app revokes Apple Mail's OAuth token. I therefore wouldn't use that as the working explanation. It's possible the two events share a cause—for example, Google asked him to authenticate again and he wound up in Gmail—but Gmail itself shouldn't evict Apple Mail.

One last zero-risk check before deleting

I'd do this:

Settings → General → Software Update

If nothing interesting appears there, then I'd proceed with Delete Account.

Apple explicitly says deleting the account here removes it from the iPhone and does not cancel/delete the actual email account. citeturn0search1 And when you put it back, choose Google, not a manually configured IMAP account. Apple specifically provides Google as a supported provider for this purpose. citeturn0search5

So after checking the software version, I'm comfortable with you hitting that Delete Account button. The competing explanations I've found either don't fit the evidence as well or are themselves repaired by getting Apple Mail a fresh Google authorization.