What’s unclear about that message?
The error states that it failed to get an address for a hostname.
No network means no DNS resolution. That results that in the application not knowing what IP is used by the outbound mailserver it should connect to.
If you aren’t concerned with the technical aspects, you can stop reading after Failed to send some messages.
[not trying to “pile on” here, but …] K9, as with most externally oriented mobile device applications, requires internet connectivity to work. Having “internet” enabled isn’t sufficient - your device has to have a data connection, which doesn’t appear to be the case here.
Some devices, e.g., a watch, can leach, (or be configured to) connectivity from another nearby device via bluetooth and continue to work (in a manner of speaking), but it doesn’t look like your phone is connected to anything else via bluetooth either.
A message you send under these conditions should stay in your K9 “outbox” until your device has suitable connectivity.