Connection to outgoing server fails

The master copy of mail, contacts, etc is on the Yahoo server. If you delete the Yahoo account from the iPhone, restart the phone, and add it back the content will be resynced to her phone.

You do this in Settings/Mail/Accounts; tap on the account to see the delete option, then after restarting the phone go back to the same place and add the account.

The VPN could very well be the problem.

Something to think about→Don't use VPN services. · GitHub

There are two legitimate purposes for using VPN:

• To allow access to a private network such as a school or business when you are not on site. 
• To allow access outside of a country with a repressive government that has restricted Internet access.

Any other use is risky, and can lead to problems like the one discussed in this thread. VPN disguises your location by making you appear to be somewhere else in the world. But you can’t control that “somewhere else”, and if it is in a location that an app isn’t approved for the app won’t work. Plus the fact that the provider of the VPN knows everything about you and your location, as well as what sites you access through the VPN. So you are totally dependent on the VPN provider’s honesty. As a start, if the VPN is free, DON’T USE IT. The provider has to make money somehow, and if you aren’t paying them then they are selling your private data to make money. But even those that charge can’t necessarily be trusted.

You don’t really need VPN when using public Wi-Fi, because all communications between your iPhone and the servers it accesses are end-to-end encrypted.

With iOS 15.2 Apple now has iCloud+ Private Relay, which is like a super VPN, except that it doesn't spy on you→About iCloud Private Relay - Apple Support

Is she using a VPN connection on the phone? Using VPN can cause issues like that.