This is related to #272584, in a way also to #271262 

At present, online/offline mode is handled globally. However, different accounts may require different connections to reach their server. 

Evolution should allow the user to associate a particular mail account with a specific connection, similar to how (sorry for having to mention it) MS Outlook handles this.

A frequent case is that of a VPN tunnel which needs to be up and running to contact a particular server. With the pptp plugin in network manager, it should be possible to determine automatically, whether the vpn connection is currently up or not. 

Another case is that of a local mail server, either in the local LAN or even on the local machine. I have an IMAP server running on localhost, which is by definition, never offline. If I could associate this mailserver with the local loop interface, it would never go offline, just because the computer is not connected to the outside world.

I don't know, which of my suggestions would need modifications to network-manager and how much of the required functionality it already provides.