When starting Claws Mail with the command line parameter '--offline', Claws may still retrieve e-mail. This is a security or privacy issue. This also results in error messages regarding network connectivity if e-mail retrieval fails. I have observed this at least when Claws was previously shut down in online mode with an IMAP e-mail account.
Claws version 3.9.2
Following a discussion on IRC, it turned out to be a packaging problem. The Claws Mail packaged version was built with NetworkManager support but did not depend on NetworkManager. The OPs system did not have NetworkManager installed. Worked around by setting use_networkmanager=0 in clawsrc