No error shown on DNS error

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App

Thunderbird for Android

App version

13.0

Where did you get the app from?

F-Droid

Android version

GrapheneOS, Android 16

Device model

Pixel

Steps to reproduce

1. Add an account with an IMAP and SMTP server on a local domain or a domain defined only within a company VPN (something like: private-mail-server.lan),
2. Change network (or disconnect VPN) to one on which the domain is not defined (causing ping: unknown host),
3. Try to refresh the message list, or to move a message between folders.

Expected behavior

The app should alert the user that the email server is not reachable, showing an error and its cause.

Actual behavior

Nothing happens and no error is displayed.
Furthermore, the failed DNS resolution is cached and causes the silent failure to persist for a while.

Logs

No response

Start by reproducing the issue with an IMAP and SMTP server on a private domain, then disconnect the VPN or change networks and refresh or move a message. Trace how the DNS failure is handled and cached; done means the app displays the server-unreachable error and does not silently persist the failed resolution.