I use offlineimap to sync my mails to a local IMAP server. In Claws Mail I configured this IMAP server (localhost) as my primary mailbox. It would be great if Claws Mail would be able to continue to connect to the IMAP server at localhost, even after I switched to offline mode. Right now I can't switch to offline mode because then Claws Mail refuses to open my local IMAP. So I have to stay always "online" which means that many operations are delayed if I don't have a internet connection, e.g. Claws Mail tries to get the GPG key for a mail I just want to open, it tries to update my RSS subscriptions, etc.
I don't think this is a good idea, because if you can connect to localhost you can connect to any host, defeating the purpose of being off-line. Once upon a time there was a MUA called Lotus Notes which had some kind of "network locations" which allowed to select which "network" from the predefined set of profiles the client was in, adjusting connections accordingly. This is what you probably would like to have, but that's also more complex to implement and to use, as you cannot use hostnames and ports, but have to use some abstract connector which is configured to some host and port in other part of the program. BTW, Claws Mail does not retrieve GPG keys AFAIK, that's done by gnupg when you have auto-key-retrieve in keyserver-options. Not sure how allowing connections to localhost in Claws Mail is going to prevent that.